Many four post lifts do not have to be bolted down. In fact, some have optional caster kits that allow them to be moved around when they are down, some even with a car on them. Four post lifts can be had with rolling jack pans on them and using something like air bladder jacks, it is quite easy to raise one end or one corner of a car, allowing wheels to be removed, brake work, etc. They make a handy work shelf for tools while you are working. Putting a car on a four post lift is relatively easy, just drive it on and up you go. Four post lifts allow the car to sit naturally for work like exhaust positioning which requires the suspension to be loaded and not hanging down. It is relatively easy to drain oil and transmission fluid with a four post.
Two post lifts allow for easy access to all of the undersides and leave the wheels hanging for suspension and wheel and brake work. Its very easy to change oil and transmission fluid with a two post lift and removal of underside components is quite easy, such as fuel filters, fuel tanks and such. The ramps on four post lifts sometimes get in the way for underside work. Two post lifts must be bolted down, requiring solid concrete floor and no cracks or flaws.
Some work can be just as easily done on either type of lift, other types of work are more difficult or impossible to do on one type of lift or the other.
Someone commented recently that after six months of owning a two post lift, you will be wishing for a four post, as its a hassle to pull the car in centered and just the right place for balance on a two post lift, as a four post, you just drive it on and if its a little off, it really doesn't matter.
I own a two post lift, and I'm starting to agree, a four post would be nice to have.....
Charles