What size wire for a 4000 amp service?

Generators are rated in volt-amps (va) and may also have a wattage rating. In most practical applications volt-amps and watts are the same.

At 240v single phase, 400 amps is 96000 volt-amps, or 96kva. I would be looking for a 100 kva generator. This is a large generator if you are considering it for your home. In most situations you identify critical circuits and place only these on the generator through a transfer switch. This allows for a much smaller generator.