power problem
 

so i just finished my manual convertion and i was driving it and when i try to gun it i lose power and it just stays at 4000 rpm's it has power throught 1-3000 rpm's but once i get to 4 it just stays there i would think its the cat cuz my freind had the same problem but idk the cat looks fine. i really need the help =/

year, sub model?

86 gxl