Obviously it's got to be fuel, spark or mechanical. From your post, nothing points to mechanical as there should be nothing magical between 4400rpm and 5000 rpm.
Fuel? Could be a fuel starvation problem. But it doesn't sound like that to me. Is it sputtering or a real drop in power? Maybe you should put a pressure gauge on your fuel rail and bring her up to 5000rpm. If you see a significant pressure drop, you know where to look.
Anyway, assuming this is a stock setup with stock FI, it appears to be electrical. At the higher rpms, the dist is spinning pretty fast and a cracked dist cap or some such nonsene can cause significant power drop over 4000 rpm. In my case, even though there was no indication of a problem with the cap, a new dist cap completely resolved upper rpm power loss.
But first of all, isolate it to fuel, spark or mechanical. You;ll save your self some time.
... And let us know more about your setup.