The differences in barrel length are several. As was already mentioned, increasing barrel length increases velocity. Increasing barrel length also increases your sight radius. This means that small errors in sight alignment are magnified with a shorter barrel. Barrel length affects the balance and "feel" of the gun also. Probably the biggest factor in handgun barrel length is ease of carry. I doubt if you will be trying to carry a medium frame .357 concealed but obviously, if you were, the 4" would conceal easier than the 6". More to the point, the 4" is also easier to carry exposed on your hip also. I often carry a handgun on my belt while spending time in the boonies and I find the longer barrels to get in the way. For example, trying to sit in the car results in the barrel sticking into the car seat and the holster tries to pull your belt around your neck. I own somewhere around six .357 handguns and really enjoy them all. If I could have only one, it would be the Ruger GP-100 Stainless in a 4" barrel. It is built well, it is accurate, it is durable, it is very strong, it is reliable, it is easy to maintain, and the size is comfortable to carry.