Can you change the barrel on a revolver from a 3 inch barrel to a 4 inch barrel?

Yes, barrels can be changed. However, unless you have a Dan Wesson that is made to have quick change barrels, this is VERY difficult for the average hobbyist to do without doing serious damage to the revolver. It takes some specialized tools, not just a heavy vise and a pipe wrench. Find a gunsmith in your area, and talk with him.