Looking for a suggestion. I have a 2.0 liter with flat top pistons (euro style), carb cam, bursch exhaust. It has the dual carb set up, idle jet is a 55, can't remember what I have for the main jet. Carbs are synced.

Engine runs fine through the rpms under normal driving. But, when I get on it, full throttle, around 4500 rpm and up, the engine starts popping and on more than one cylinder.

Any idea on what might be the issue?

Number one I'd say is your linkage is not opening both carbs at the same time/rate.
Poor linkage design is the most common cause of poor running carbs.

OR.... leaking intake manifolds
OR.... white deposits in the carbs from running pump gas with ethanol (which results in clogged jets)

I switched from a hex linkage to tangerine racing wire line linkage and all the popping went away.

thanks for the comment - I'll also look to make sure my vacuum advance hoses are on properly

What type of distributor are you using that requires a vacuum signal? Are the carbs balanced at idle or when the idle speed arms are off the stops?

At idle, the carbs are all pulling the same air flow number on the synchronizer tool I have. While on the stops.

As for the distributor, the car came with one out of 1.9l water-cooled vanagon. The -009 one was ditched per the PO for the 1.9l version as he claims "its the one that rev's higher and the advance curve seemed to be more aggressive and the angle of advance is changing over a border range of revolution (vacuum AND centrifugal advance). The good thing is that the 1.9l distributor comes with VW OME hall sensor or opto electronic sensor and electronic ignition module." - his email words

I always set the synch at idle, THEN I synch it again where the snail is reading 7-10 range which is above 1500 RPM.
My "calibrated" tool is a popsicle stick jammed between the carb lever and the stop on the 'master' carb (side with the throttle cable if using the cable style).
Ideally it should get checked at 3500 but that is beyond the standard snail synchro range.

Turning ONE idle speed screw in will raise the idle and the linkage will be in an 'operational' state. I've never heard of the use of a vanagon dist in a t-4.

wished all things were so easy to fix. After checking vacuum hoses for leaks, ensured the throttle linkage was all set correctly, and checking the synchronizing at different rpms, it turns out the #3 spark plug boot was not secured on the plug. Noticed it was out a bit too far. With the rubber cap on the plug boot (for minimzing fan air pressure leaks), the pair became somewhat of an umbrella. The plug boot was on the spark plug and contact was fine or at least worked as long as I drove normal. But once I cranked u the rpm, the fan pressure moved the plug/boot far enough up the plug that contact was lost, thus #3 plug wasn't firing. Attached boot securely and runs like a top

Good job!