Ammunition performs to its specific design parameters, not universally based on caliber. A given load will perform differently in different guns based not just on barrel length but a variety of other factors that can vary even within the same model.
Everything else being equal, the same load will have less velocity out of a 3" versus 4" barrel -- this can be good, bad or indifferent based on the load's design parameters.
Point is, don't automatically think of less velocity as being bad, think of proper velocity for the round's design and whether or not it's met when fired from a specific revolver.
Case in point: Speer's Gold Dot Short Barrel .357 load will have less velocity out of a 3" barrel than a 4", and this is preferred because the load's design is to achieve a certain penetration depth and expansion at a particular velocity range likeliest in barrels 3" or less; again, all things being equal, with this load at 4" or greater the bullet will be overdriven and not perform to spec.
So don't think purely in terms of velocity, think in terms of choosing ammunition for its designed performance parameters and make certain they're met in your particular revolver.
