Sometimes people mistakenly think there's a problem with a carb on Onan 4000 generators. Be sure to ensure the fuel pump is pumping fuel to the carb by pulling the hose off the carb and watching for adequate flow. A faulty electric fuel pump can often only show symptoms or act up after it is warmed up. Sometimes it may run for 10 or 20 minutes and just shut down. Check for fuel flow immediately upon stalling if this is your symptom. Mine would shut down after 20 minutes and not restart for awhile. I pulled the hose off the carb immediately and noted there was no fuel in the line and it would not pump fuel while holding the start switch on either. If you are mechanically savy you can disconnect the hot wire the positive wire (red) leading into the fuel pump and connect it with 12 V and see if it pumps fuel out the hose also, but be careful to capture the fuel coming out the hose). Once cooled down it pumped fuel in a weaker than normal stream. Replaced the fuel pump (and new filter while at it) and everything worked fine again on my 1995 Onan. Much stronger fuel stream noted too from the hose. Don't run it very long without the side panel on it also, this helps with directing air flow across the engine properly.

Check the current draw from the machine
being a 4000 watt machine if you are drawing 4000 watts it is overloaded as that is the surge power available not constant power which is around 3500 watts
that would account for the requirement of fans to keep it cool
even if It is not 4000 watt what ever capacity it is it is running in an overload situation