I just recently bought an 03 600 gsxr and the bike is stretched 4 inches. Now normally, stretching requires a new swingarm or the adjustable swingarm, but the bike I have was stretched by adding 4 inches off of another swing arm. My question is, is it safe to weld on another piece onto the existing swingarm? I thought it wasn't safe to modify the frame, subframe, or swingarm.

If welded/set-up correctly ,Yes.

welding onto the existing strecthed part of the swing arm or are you going to break off the extension and add a whole new extension.