I have a generator with a 4 wire plug and want to backfeed it into my 3 wire plug from my air compressor. Do I run four wire out of the generator and then combine the nuetral and ground together at the 3 wire plug?

Hi,
No you would typically put the ground to the frame of the machine and then run a ground from the machine to an "earth" ground. This is for safety. The neutral and ground "should" be at the same potential, but are often just close with the neutral being slightly "off" ground.
Peace,
Clarke