"There is something wrong with an engine -- or associated systems -- that feels like it's struggling at 4000, although it seems to be a common complaint."

Seems like there's been more than a few people mentioning that lately, hasn't there?

It's probably not mechanical (i.e. bottom end, cam, cylinder head) in nature, otherwise you'd have some other symptoms (uneven compression, uneven running at 4k WOT). It must be related to either the induction, the ignition, or the exhaust. Although you can't completely discount the occasional cam gear installed a tooth off scenario...

Induction: Are the carbs jetted properly, do they get enough fuel flow from the pump to keep up with the engine's consumption at WOT. In other words, are they going lean when pushed hard? Is there any sort of unusual obstruction in the intake - clogged air filters, stuck secondary butterfly valves (get rid of those!)? If they are SU's are the pistons free to move up and down? Are they in decent tune?

Ignition: Is the advance stuck? With a timing light, watch the timing at idle, then rev it up to 3 or 4K, does the timing advance 20+ degrees? Is it set properly at idle? Is the spark nice, fat, and blue, or weak, thready, and reddish? Are the plungs in decent shape and have at least reasonably close gapping (I've never noticed much of a difference in gaps). Is the points gap set properly (do this first, otherwise you'll have to redo the timing) - if too wide or to narrow the coil won't get proper dwell, and improper dwell seems to affect higher RPM running more than low. Are the lobes the points run on worn down - imght be impossible to get proper dwell if so. (PS - Fram what I understand - dwell refers to the percentage of time on and off the coil sees - the points alternately ground and unground the current flowing to the coil - it needs enough time grounded to properly energize and enough time ungrounded to properly discharge - too little of either and you don't get a good spark). Issues with the ignition are often sneaky and can pass themselves off as other things. I could have SWORN that the weakness and wavery feeling my PV used to have up past 5000 was carb related, but it went completely away when I put on a Crane ignition and a Mallory coil. Ignition systems are NOT graded on a pass-fail basis. They don't either provide an ignition or a miss - there are quantifiable differences in the ignition they provide.

Exhaust: Not much here in terms of holding a stock motor throttled back to 4K. Even before i got rid of the puny original single downpipe system on the PV (seriously had a 1 1/4 inch ID!) my B20 would pull past 4K. However, a partially blocked muffler can do it - they can rust and partically collapse internally sometimes (depending on the type of muffler). And if you have a single downpipe manifold and a truly puny exhaust system you will notice it up past 4500 - 5000 rpm or so. Unless you are building a 180+ beast a dual downpipe cast iron manifold is sufficient, IMO.
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I'm JohnMc, and I approved this message.