Domzilla, it's not so much a difference in velocity as that it pushes the bullet into a more favorable performance envelope for expansion. Most .38 2"-barreled snubbies shoot +P rounds at 800-850 fps. This is very marginal for expansion, and if the hollow point of the bullet gets plugged with clothing material, expansion may be limited or non-existent. A 4" barrel gives an extra 150-200 fps to that load, and the additional velocity means that the bullet will hit harder, forcing more material into the hollow point, and promoting expansion.
You can see the same thing at work in the famous .357 Magnum 125gr. hollowpoint, traditionally doing about 1,400 fps out of a 4" barrel. At the time this load rose to prominence, in the late '70's and early '80's, it did NOT have a particularly good or efficient hollowpoint bullet. However, it had so much velocity that even a not-so-good hollowpoint was very likely to undergo explosive expansion, and even fragmentation. It built its reputation on its velocity. Today, with much better designs in hollowpoint bullets, a 9mm. shooting a 124gr. +P hollowpoint will produce similar bullet performance on target, even though its round is moving a good 200 fps or more slower than the famous .357 Magnum equivalent. Back in the '70's or '80's, the 9mm. had a miserable reputation as a "stopper" compared to the .357 Magnum - but that's changed...
In .38 Special, the newer loads designed specifically for snubbies, like the Winchester 130gr. SXT JHP, have very good performance indeed, even at a velocity of 800-850 fps. They're streets ahead of older-generation loads, even the good ones like the classic "FBI load" of a 158gr. LSWCHP at +P velocities. However, push the FBI load out of a 4" barreled revolver, and its performance improves to match that of the newer-technology rounds shot from snubbies.
(From a snubbie, at least.)
