What does re4 mean in the air National Guard?

RE4 is a reentry code given when a service member is dicharged. Some branches get a little more specific (they'll have something like RE4A, for example to specify an exact reason), but an RE4 in all branches means they're not recommended for reentry into the military. This could be due to a Bad Conduct/Other Than Honourable discharge (which is usually the case), but it can possibly be for reasons which aren't as malicious, as well.