If the Generac is rated at 4000 watts, it should be able to run one roof AC unit and the microwave at the same time. Start generator, start AC fan, let run for a minute, set control to cool and thermostat to 72 degrees and listen for compressor to run, let run for a couple of minutes, then try heating something in the microwave. If you have a line voltage meter check if voltage is in the green or 110 volts with AC and microwave on. If voltage is low, Generac may need voltage adjustment or other repairs. Run AC alone and check line voltage is in the green, if not, don't run AC until you get generator fixed.