You're more than welcome to adopt the idea. That's what GJ is for.
If I did it again today, I'd do it a bit differently:
I'd drill the foot and leg at one pass before separating them. When the feet are cut loose from the leg, mark them as a pair. No matter where the hole is, or how parallel it is to the centerline of the leg, the foot will sit level on the floor.
As for the collars, they'll work best with milled legs. On my bench I know that all 12 legs are within a few thousandths of each other dimensionally. It will be easy to cut 24 faces and 24 sides and be relatively assured that I can have a good fit. Frankly, the way the design grew, they're not really necessary, but I think they'll look nice...
So, for your situation:
Cut the end of the 4x4 first - nice and square.
You can skip the guide block if you can hold a drill nice and square to the end. Drill deep enough to create the required pocket in the leg for the leveler thread. If you want to use my idea to thread the foot with the leveler, then make the hole undersized, and bore out the hole in the leg once you separate the foot from the leg.
Personally, I wouldn't use 4x4's for the legs. At least down south, 4x4's are some of the worst pieces of lumber you can buy. At my local big box stores, they're only available treated, and treated wood does nasty things to woodworking tools if left in contact. Also, treated wood is generally sopping wet when you buy it. As it dries, an otherwise straight and true 4x4 can twist and gyrate in ways that would make a salsa dancer envious. Not something you want with an assembled workbench.
I've had 4x4's installed in fences practically tear themselves loose from the rails as they twisted and contorted while drying out.
If you have the means and equipment, I'd mill some 2x4's and glue them together.
If not, try this on for size. But first, a little lesson in grading lumber (pardon if you already know all this).
There are many things that can be wrong with a piece of lumber. Most lumber being sold today exhibits one or more of the following flaws. I've seen pieces that had all these issues and more.
There are some definitions that make no differentiation between Bow and Warp. For the purposes of this discussion, I'll stick with my definitions.
You can identify most of these flaws in lumber before purchase, and predict when otherwise straight lumber will manifest these flaws at a later time. Sighting down the edge and face of a board is the quickest way to see Warp, Bow, and Twist. Checking is obvious splitting in the end, and wane is also easily visible, as is cupping, although cupping is harder to see in smaller lumber. Large knots that protrude through the edge of an otherwise straight board will cause the board to warp as the knot dries out. Twist is harder to predict.
To continue, for 2x4's that aren't milled, try this.
Get a pair of 2x4's that are free of twist and warp (straight when sighted down the edge). What you do want is boards that are bowed, preferrably evenly and not too severely. I think an ideal bow would be 1 to 2" along the length of an 8' stick. (Hold one end down - the other end should be 1 to 2" above the surface the held in is abutting.)
Take 2 boards and put a good quality wood glue (I like Titebond II) on the convex side of both boards. Use a nice thin coat, an old credit card with notches cut in the edge makes a dandy spreader.
Do not cut the boards to length yet - work with the full lengths.
Clamp one end of the boards together. Avoid using any sort of fasteners (screws or nails), as they'll just give you grief later.
With the convex sides facing each other, you probably won't need clamps in the middle - one on each end will suffice.
Take the time to correct any difference in the edges of the boards where they don't align - use a block and clamps to pull the boards into alignment. Put blue tape or use wax paper between the blocks and the boards to prevent them becoming part of the leg blank.
Clean up any glue squeeze out once it's had time to cure for 30 minutes or so - easier now than when the glue dries.
Let the blanks dry overnight. What you have now is two boards that by the grace of glue are more like one board. The flaws in each, directed against the other, cancel out the flaw and produce a blank that will stay dimensionally stable. They'll be 3" thick and 3-1/2" wide. Not as big as a 4x4 (which is 3-1/2" x 3-1/2") but more than sufficient for your application.
This idea does have a drawback. Drilling for the feet will be more problematic, as you'll be drilling into the area between the two boards where they're glued together. This will tend to make the drill bit walk or draw. If you use my earlier suggestion and drill the foot and leg before separating the two, any walk or draw won't be an issue.
If you're building a bench for pounding, you need something to prevent the legs being driven through the top of the bench as you hammer. Just bolting the legs to the apron (the rim of the table that the top attaches to) isn't sufficient. I'd cut a 1/4 to 1/2" deep rabbet in the face of each leg where it attaches to the apron. That way the apron has something solid to sit on and the hammering forces will be borne by that ledge, not by the screw that holds the leg to the apron.
For the leveler screws, you'll need to either use nylock nuts, or double-nut the leg end (assuming you'll thread the foot like I did). The pounding action will cause the nut to creep on the thread and change the length of the leg in doing so.
Also - a pounding workbench needs to be solid from table to floor. Any gap at all between the top and the apron, the apron and the leg, the leg and the floor will allow the frame to bounce when you hammer, stealing much of your hammering energy. Make the aprons nice and true, and bolt or screw the top down to the aprons to make sure there's no spring in the assembly.
Small ideas, long post - it's what I live for...
Good luck - post your completed bench up for me and others to see and admire.
If I did it again today, I'd do it a bit differently:
I'd drill the foot and leg at one pass before separating them. When the feet are cut loose from the leg, mark them as a pair. No matter where the hole is, or how parallel it is to the centerline of the leg, the foot will sit level on the floor.
As for the collars, they'll work best with milled legs. On my bench I know that all 12 legs are within a few thousandths of each other dimensionally. It will be easy to cut 24 faces and 24 sides and be relatively assured that I can have a good fit. Frankly, the way the design grew, they're not really necessary, but I think they'll look nice...
So, for your situation:
Cut the end of the 4x4 first - nice and square.
You can skip the guide block if you can hold a drill nice and square to the end. Drill deep enough to create the required pocket in the leg for the leveler thread. If you want to use my idea to thread the foot with the leveler, then make the hole undersized, and bore out the hole in the leg once you separate the foot from the leg.
Personally, I wouldn't use 4x4's for the legs. At least down south, 4x4's are some of the worst pieces of lumber you can buy. At my local big box stores, they're only available treated, and treated wood does nasty things to woodworking tools if left in contact. Also, treated wood is generally sopping wet when you buy it. As it dries, an otherwise straight and true 4x4 can twist and gyrate in ways that would make a salsa dancer envious. Not something you want with an assembled workbench.
I've had 4x4's installed in fences practically tear themselves loose from the rails as they twisted and contorted while drying out.
If you have the means and equipment, I'd mill some 2x4's and glue them together.
If not, try this on for size. But first, a little lesson in grading lumber (pardon if you already know all this).
There are many things that can be wrong with a piece of lumber. Most lumber being sold today exhibits one or more of the following flaws. I've seen pieces that had all these issues and more.
Wane - this is where the edges of the board aren't square and can even show evidence of bark. These boards come from the edge of the tree.
Twist - like a barber-pole, the looking down the flat edge of the board, the far end of the piece isn't parallel to the end you're looking at.
Cup - across the wide dimension of the piece, it's not flat.
Check - splits in the end of the board.
Warp - this deformation occurs along the thin edge of the board, across the face. A warped board, when stood on end with the broad face facing you will turn left or right.
Bow - this deformation occurs along the face of the board, along the edge. A bowed board, when stood on end with the broad face facing you will curve towards or away from you.
There are some definitions that make no differentiation between Bow and Warp. For the purposes of this discussion, I'll stick with my definitions.
You can identify most of these flaws in lumber before purchase, and predict when otherwise straight lumber will manifest these flaws at a later time. Sighting down the edge and face of a board is the quickest way to see Warp, Bow, and Twist. Checking is obvious splitting in the end, and wane is also easily visible, as is cupping, although cupping is harder to see in smaller lumber. Large knots that protrude through the edge of an otherwise straight board will cause the board to warp as the knot dries out. Twist is harder to predict.
To continue, for 2x4's that aren't milled, try this.
Get a pair of 2x4's that are free of twist and warp (straight when sighted down the edge). What you do want is boards that are bowed, preferrably evenly and not too severely. I think an ideal bow would be 1 to 2" along the length of an 8' stick. (Hold one end down - the other end should be 1 to 2" above the surface the held in is abutting.)
Take 2 boards and put a good quality wood glue (I like Titebond II) on the convex side of both boards. Use a nice thin coat, an old credit card with notches cut in the edge makes a dandy spreader.
Do not cut the boards to length yet - work with the full lengths.
Clamp one end of the boards together. Avoid using any sort of fasteners (screws or nails), as they'll just give you grief later.
With the convex sides facing each other, you probably won't need clamps in the middle - one on each end will suffice.
Take the time to correct any difference in the edges of the boards where they don't align - use a block and clamps to pull the boards into alignment. Put blue tape or use wax paper between the blocks and the boards to prevent them becoming part of the leg blank.
Clean up any glue squeeze out once it's had time to cure for 30 minutes or so - easier now than when the glue dries.
Let the blanks dry overnight. What you have now is two boards that by the grace of glue are more like one board. The flaws in each, directed against the other, cancel out the flaw and produce a blank that will stay dimensionally stable. They'll be 3" thick and 3-1/2" wide. Not as big as a 4x4 (which is 3-1/2" x 3-1/2") but more than sufficient for your application.
This idea does have a drawback. Drilling for the feet will be more problematic, as you'll be drilling into the area between the two boards where they're glued together. This will tend to make the drill bit walk or draw. If you use my earlier suggestion and drill the foot and leg before separating the two, any walk or draw won't be an issue.
If you're building a bench for pounding, you need something to prevent the legs being driven through the top of the bench as you hammer. Just bolting the legs to the apron (the rim of the table that the top attaches to) isn't sufficient. I'd cut a 1/4 to 1/2" deep rabbet in the face of each leg where it attaches to the apron. That way the apron has something solid to sit on and the hammering forces will be borne by that ledge, not by the screw that holds the leg to the apron.
For the leveler screws, you'll need to either use nylock nuts, or double-nut the leg end (assuming you'll thread the foot like I did). The pounding action will cause the nut to creep on the thread and change the length of the leg in doing so.
Also - a pounding workbench needs to be solid from table to floor. Any gap at all between the top and the apron, the apron and the leg, the leg and the floor will allow the frame to bounce when you hammer, stealing much of your hammering energy. Make the aprons nice and true, and bolt or screw the top down to the aprons to make sure there's no spring in the assembly.
Small ideas, long post - it's what I live for...
Good luck - post your completed bench up for me and others to see and admire.