If the 4 speed is a truck type trans, the first gear is gear is real low, called granny gear. It is not good for much unless you are pulling stumps or something like that. The other ratios are about the same as the 3 speed. The only advantage would be the direct floor shifter, but if your linkage is good the 3 speed is fine. It would hardly be worth the swap. If you put a car type 4 speed in it you would have the advantage of closer gear ratios. A 5 speed with o-drive would give you a good highway gear.
