Hubba: i'm guessing a 5 inch wide jaw and maybe 80-90 pounds? there is a line under the static jaw that could be a crack. is it? is this your vise or just something you saw at a sale or somebody's garage?
RL: my wilton C2 had 5 inch jaws and only weighed 112 pounds. i'd be surprised if your 4 inch wiltons weighed 100 pounds. more like 60 because the 4.5 inch wide jaw Wilton C1 only weighs 65 pounds.
if yours are not mounted you might want to weigh one or both. or look up on Wilton's web site and see what a new one weighs.
RL: if you do have a 100 pound 4 inch Wilton can you post a few pictures? my 6 inch Wilton Tradesman only weighs about 70 pounds or maybe it was 75.
ZK: I'm pretty sure the newer 6 inch wiltons have that jaw style, but you are right the older one's jaws bolt through the back. i'm thinking the one the OP posted is a 5 inch.
I have 3 generations of 4-inch Wilton swivel base vises, and only the later Schiller park version weighs over 50 lbs. And it's about 52-55 lbs at most.
When you say "Both my 4" Wiltons", did you mean TOGETHER?