Need Help With Engine Diagnosis

4000 is quite far up in the rpm band.
Question is; does happen under WOT only, or under all throttle settings?
If it's gas pedal related, it's almost a sure thing that it is a carb issue.But if it still happens at part throttle, it could be mechanical.
If it just quits reving at 4000 no matter what you do with the pedal, and you don't have a rev-limiter, then it's most likely to be lifter pump-up. This is easily proved by immediately putting it in neutral,and allowing the engine to return to idle.If the lifters are pumped up, it will stall,and be extremely hard to restart.
But if, after backing off the throttle, she will motor through 4000, and continue to 5000 and beyond, then it is very likely to be a fuel issue. The very first thing I would then do, is a fuel volume delivery test.I would fill the float bowls, and then plumb the fuel delivery line straight into a mimimum 2Qt container.Then I would time how long it takes to pump 1Qt (or about)of gas into it at idle.A good system will pump that in 1 minute. An excellent system will pump that in 30 seconds. If it does that, I would move to the carb.
To find out which circuit is guilty, I would disable the secondaries, and try again.Obviously, if it now works fine, then the secondary side needs a look-see. But if it still pukes, then it's very likely to be in the power circuit.
I'm a little concerned about the earliest comment,about requiring to slip the clutch out more than you remember.Between this and the other, I can easily see a faulty power circuit; either a stuck piston, or oxidized rods, or some other fault.
You don't mention fresh gas tho, so even tho Seafoam has stabilizers in it, it cannot bring stale fuel back from the dead.Oxiginated gas goes stale very rapidly when left in an open-to-atmosphere container.The ethanol portion evaporates, leaving the heavier gas behind, which,over time becomes a syrup,and eventually turns to a solid varnish.This can begin to be noticeable in as little as a week in lawn and garden equipment, and certainly noticeable in your V8, in 2 weeks.The length of time that the entire process takes, depends on how much fuel you start with.If you started with a half tank two and a half years ago,most of it will be varnish or syrup today, unless it was Seafoamed back then.However,Seafoam has a limited life as well, and after a year, it loses a great deal of it's effectiveness.So if the fuel ain't FRESH, get rid of it.The crusty white powder in your carb is the residue left by the evaporated oxiginated gas.That powder will be everywhere in your system,everywhere that liquid gas was not. If the tank was allowed to evaporate to empty, then it is everywhere (not so much on galvanized surfaces and more so on aluminum),But worse is that the pick-up sock will be varnished up, cuz it sits at the lowest point.
Good luck