Hi, and welcome to THR!
The answer is not exact, nor always the same. No two guns will fire two identical cartridges at exactly the same velocity. In general, though, the slightly less than 2" barrel of the 642 will produce something around 70-100 fps slower velocities than a 4" barrel would. (Usually we expect to see about 35-50 fps per inch of loss, with a shorter barrel handgun.)