Mike,
Start off with the basic diagnosing.
Pull the spark plugs and read them...While out you could do a compression test.
Check your timing and if it good you could leave that timing light hook up and everything routed out of way fan,header etc...
See if you can position your timing light so it is under your wiper blade so it is point at you and a zip tie holding the trigger on....Test drive it up to your 4000 rpm point and when the problem occurs does the spark become erratic....
Put a vacuum gauge on it,how much does it pull at idle ,does the needle fluctuate...
What kind of carb are you running?
You could balance test the motor by pulling plug wires one at a time while running to see how much rpm drop each cylinder does when you pull spark from it.
If one or more cylinder have little to no drop in rpms,that is where you will go to further diagnose.
For example,your primaries are feeding different cylinders,if one side of the primaries is not operating properly certain cylinders would be affected.
A vacuum leak at a particular port in the intake would affect that cylinder.
If it is running rich,I do not suspect it to be a vacuum leak...
I suspect a carb issue unless you are not getting a strong spark...
What ignition system you running?
If you bent a pushrod,I feel that you would notice that dead miss...
Valve adjustment would not hurt though...
Hope some of this helps,
TXDart