Onan 4ky, if it will start it will die after 30 minutes. Then it will not start at all. No fire at plug.

SOURCE: I have a 1999 Onan Generator start/stall issue

Check your oil level. The unit will stay running as long as you have the switch depressed. When you get enough oil pressure a switch closes & keeps it running.

SOURCE: onan 30 DDA dieasl john deer gen runs great for

its seising up ,the oil is not getting round properly,blow out all pipes with a compresser,or its over fueling

SOURCE: 3.3lt engine, the engine will

It sounds like your fuel pump is going bad.

SOURCE: onan generator dies after 2-3 hours of running ,

This sounds like overheating. These small generators don't have overheating protection, but if a low oil pressure or low oil level is present it may be the reason for stopping.

Check the oil level in the starting conditions and again when it stops. It may become under the "stop" level and after some time the precipitated oil has reached the "start" level. The same may be with oil viscosity and oil pressure.

Try to add some oil or change the oil with bigger viscosity.

SOURCE: starts easily, run for 5 to 20 minutes and then

If the fuel filter (strainer) is OK, carb gives enough fuel for the load you apply, air filter is dry and clean, your problem may be the low oil level protection.
Check if generator is at level position.
After the oil is distributed in all the engine its level may go less than the measuring sensor level. Try to put some more (not much) oil. 
If this will not help your measuring sensor may be bad and switch off when the oil gets hot.