At 4K, your bike is on the jet needle. And your carb is probably a CV (constant velocity), as my 79 Suzuki is one of the last years before CV carbs. The jet needle is pulled out of the main jet by differential air pressure. It's probably sticking. In other words, the carbs are dirty. Specifically, the diaphragm/piston and walls, and possibly one or more air bleeds.
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