Geez.....looking at the video, it is starting to look like a cam timing or distributor timing issue to me. Have you taken a timing light to it to see exactly what your timing is doing at various rpm's? I'd try that first (easiest). 2nd, if the timing all looked good, I think I'd pull the timing cover and see what I see. It's not that major of a job.

OK, just to be clear.... is this a relatively new build or recent overhaul OR did the engine run great for years & now all of a sudden it won't rev past 4 grand?