81 kz750h Cylinder 4 no spark after 4k rpm

Hello
New to the Forum and new to Street Bikes .
Hello Fellow KZriders !
I have a 1981 kz750h ltd. i bought it For a pretty good price and it is my first Street bike! when i got it i was having issues with cylinders 2-3 not firing and traced the problems to the pickup coils . i had replaced them with a Dyna S system and the bike had been running Great for about 2 months and i have put a few miles on it since then . Recently i was on the Hiway going to work and i lost power . when i got the bike home and was diagnosing things. i found that On Cylinder 4 i was loosing spark at about 4k rpm Timing light would just go off when revs went over 4k below that the Bike runs fine and starts great. i have tried to switch the coils over and still have the same issue . all other cylinders seem to be fine when reved out.
any tips or checks i can do to help diagnose the issue would be helpful before i start replacing unnecessary parts .
Thanks