Kinda depends on what it is worth to you, and whether there is anything else that will need to be fixed. At this point, unless there is something spectacularly valuable about this car, you are basically looking at a question of how much are you willing to spend to keep the car running. (i.e. it's unlikely you will expect to get repair costs back in increased car value.)

Re: air conditioning, how bad is the leak? If it's leaking slowly, then you may be better off just recharging the system. $4000 buys a LOT of freon. However, if it is leaking rapidly, and you live in a place where having reliable A/C is important, and if you feel bad about venting $4000 worth of CFCs into the air, maybe you should have the system fixed.

Re: head gasket, are you loosing coolant? How much? Does the coolant look oily? I would get a second opinion, the engines in these cars don't really have problems with head gasket leaks.