According to Onan, the 4000 "can operate one air conditioner with power to spare" and the 6500 "operate two air conditioners with power to spare"
Actually it depends on the air conditioners. If you have those new high-efficiency Colemans that take 10A to run and 15A to start, then the 4000 could probably start two of them at the same time with virtually no power to spare.
The 4000 gives 33A, enough for a 20+A surge of one traditional RV AC through its breaker with maybe 10+A to spare for other things.
The 6500 gives 54A, enough for two 20+A surges of two traditional RV AC through each of their breakers at the same time, with maybe 10+A spare for other things.
In both cases, 4000 w/1AC and 6500 w/2AC, the "power to spare" is barely enough for a 1000W cooking power microwave (about 12.5A when the magnetron is on).