Onan Microquiet 4000 making grinding noise, won't start.

Valves probably need setting. If there not set right or if oil is to heavy or even a vapor lock can make it hard to turn over and gear teeth start slipping and making a chattering sound.
What is your full model number with the speck letter and I'll post the operator and service manual and we'll look at that code 4.
How cold is it wher your at. You also need mm to check battery volts.