Onan 4K generator starts, runs 4 minutes and shuts down

As others have said:
1 - check the gas in the main tank.
2 - hand over hand on the fuel line from main tank to carburetor. I'll bet you find another filter somewhere.

If the above turns out fine, then run a line from the pump/filter on the generator to a can of known good gas. If it runs longer than 4 minutes, the problem is in the line to the main tank. There may even be a clogged filter INSIDE the tank on the pickup tube for the generator line.