Oh yeah, back to the OP's question! 4 foot HAM antenna? HF? VHF? UHF? Multi-band?
Best place is always in the middle of the roof, no matter what band its for, antennas work best with a ground plane centered beneath them.
You can get away with an edge mounted antenna, but you will notice your antenna becomes a bit more directional, and the side where most of your ground plane is at becomes attenuated in signal.
Having said that, you could make one of the easiest temp mounts ever, just get a 2" steel receiver for your trailer hitch mount, don't put a ball on it, mount it upside down so the flat part is facing up, and stick your mag mount right to that! It will work, and I assume that antenna is capable of being tuned, so set it up and get the SWR down as low as you can.
Other than that, check with the other posters about the other mounts they were talking about.