Could you tell us what you have done with your bike mechanically, so far? (ex: have you rebuilt the carbs, checked valve clearances, etc)
I'm new here as well - it's late & I'm going to take a venture and throw some thoughts your way without writing War & Peace.
Half-Caf mentioned the air/fuel mixture, that could be a culprit. I'd be inclined to think of ignition/electrical as well. Is the ignition advancing properly (I believe it does advance
, and somewhere near 2500+/- RPMs - could be off)? Although you state that you can run through gears 1-3 with no problem (same RPMs at shift points?)...
What's the general state of tune?
Could there be a chance the petcock or even tank may have some gunk impeding the flow of fuel? (You can get an idea with a clear inline fuel filter). With this, may run at lower RPM, but when you hammer the throttle, it may fall flat. you mention that you'd checked that all 4 carbs open up evenly...assuming your bike has some mileage on it, an "eyeball" carb sync (egads!) or even a "bench sync" might get you "running", but not very efficiently (or very well, depending on the carbs). A manometer (carb sticks) or gauges (vacuum) w/ adapters will really help in the long run. I've seen times where a single carb waaay out of whack has caused problems very similar to yours.
Many questions, I know - just lending a hand. I hope I'm not way off base - with some more background, one of us here can point you in the right direction.
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