Hmm. Interesting.... If you take a look at my 48 IDF tread, you will see i've experienced the same thing: Hesitation around 4000 rpm....
As I discovered, also running a large front intake scoop, closed at the rear, the carbs were ice cold ( and so was the engine) last night running with an AF meter.
I stopped to warm the engine,
oil got into 70 (centigrades) and then I took off; full throttle in 2. and 3. (7500 rpm- 180? km/h?) and then 4. .... near 4000 and started misfire/ unfire? and behave really strange. Then back in 3. and same thing happened.
I took some carboard and put in front of radiator and engine temp got up to normal. Next run, then in 3. everything worked just fine; 100% the engine ran much better than the "cold run" earlier on.
But what caused the misbehaviour? E- tubes? Icing?
Runes.