ATF+4 will help the transmission. It deals better with heat than ATF+4. I had a low mile 1992 New Yorker with rough shifts in stop and go traffic in the summer. Out with the still clean ATF+3 and in went ATF+4. Gone was the rough shifts.Especially that A604 transmission. If it works now, DON'T TOUCH IT, period. As long as the fluid is bright purple (it may still have the ATF3, you can safely top off with ATF4 if it needs any, it shouldn't unless there's a leak), leave it alone.