For 400KVA at 480/277, I came up with 481 amps. At 208/120, it's 1111 amps.
Quite likely, it's a 10 lead gen, meaning wye connection only. If it's connected as a series wye, the output will be 480/277, or 416/240. Depends on where the regulator is set. If it's connected as a parallel wye, it'll be 240/138, or 208/120.
A 400 amp breaker is a tad small for this gen, but there's nothing wrong with it. As far as wire size goes, it depends on whether it's copper or aluminum, single or parallel, etc. At 200', voltage drop won't be much of an issue at 480, at 208 it might be.
To give an accurate size, we need more information. In addition to the above, where is this 400 amp breaker located? Is it on the gen itself, or 200' away, at the load? Or maybe there are two breakers, one on the gen, and one at the load.
Rob.