onan 4000 gen dies

Glad you got it going.
Just another sometimes overlooked item that might help someone else-
fuel tank needs to be at least 1/4 full.
A safety feature to keep from running out of gas when parked.

There is a few You tube videos about exactly what you were describing . Runs fine for about 20 minutes then dies.
The videos were saying the dipstick on the 4K gen is off. Too much oil , they go into detail about taking a turkey baster and sucking out a few oz's.

Symptoms : Runs fine for 20 minutes shuts down , will not restart . Let it sit for 20-30 minutes fires right back up only to die again after 15-20 minutes run time.

Alot of people commented saying that fixed the dying problems.
Thought what the heck , I tried it , nope still did the same thing.

Wound up replacing the fuel pump and fuel filter.

With no instructions and being in a tight place , took me an hour and a half to do it, well put the module back in tightened it down to realize I didn't fully tighten down the fuel pump retaining screw/bolt and let a wire harness drop underneath the module, so basically did it twice having to remove the module housing plate to get access to the loose screw/bolt and reroute wire harness.

Now that I know how to do it , could do it again in a half hour or less.

I paid right at $100. for the fuel pump, fuel filter and some fuel line.
RV place wanted $250 to replace the pump.

Changed the oil and put at proper level .
Ran it for 5 minutes, turned the A/C on and let run for 30 minutes .
Turned A/C off let run another 5-10 minutes ran perfect.
Have done this a couple times now, so far so good...

Thanks for updating. It helps everyone

Ok I fixed it ! replaced the fuel pump again.....
seems onan has a problem with fuel pumps? Thankful I purchased it
Local and not online .Called the onan dealer and told him my
problem he replaced the pump for me and that did the trick.
So I guess I bought a carb when mine was just fine, Thanks for
all the help guys !

Are you using the right procedure to find the fault code ?
Retrieving an Onan fault code

Before you buy anything else remove the connector from the card and check for and corrosion. Reinstalling it may give the terminals new bite. If that fixes it remove battery power and clean the terminals well. Check any other connections. You might also to bypass the oil shutdown sensor for testing

had the same problem and I did the same first three items you mentioned. At the time the Cummins Tech indicated that if those items did not resolve problem, then the next item to look at was the PC control board which of course controls the operation of the fuel pump.
Those first three items cost me approx. $600 including labor. For your info the control board is approx. $348 plus labor.
Wish you luck.