No, you don't have a "neutral." Even though ground and neutral are bonded together at the main box, they are then treated as "different" throughout the rest of the system.
Sounds to me like your colors are
black--one hot leg
white other hot leg
green ground
Now if you are going to hook this to the generator, we need to see the REAR of the generator plug.
You should have two dark brass screws, (hots)
One light/ silver screw (neutral which you WILL NOT use)
green for your ground.
The ground prong on a 4 prong connector is almost always the largest, most oddly shaped. IF ANY DOUBT use an ohmeter from the generator metal frame to the connector and see which one is ZERO ohms to the frame.
Make sure the connector is rated for the compressor current. They will have an amperage rating right on the connector (15A, 25A, etc)
Some light reading about connectors:
http://www.hubbellcatalog.com/wiring/catalogpages/section-b.pdf
http://www.frentzandsons.com/Hardware References/twistlockplugandreceptacle.htm
You also need to make sure the generator will handle the STARTING CURRENT of the compressor. DO NOT try to run it with a long extension cord
