Like others have said, 4 hrs in ATL is not a lot of time at all. A MARTA ride into Five Pt (interchange where all the trains meet) is about 15 mins and probably another 10 mins or so to the major destinations. Train normally runs every 10-15 mins depending on time/day of the week. Including waiting time, I think you are looking at 1.5 hr for traveling in and out of town. Add the security hassle in ATL which can be anywhere from half an hour to longer than an hour, I dont think you have much time left for anything at all.
Do you have gold or better status with any airlines? I believe you can use the 1st class/elite security lines if you are a gold or above. This should cut the wait to about 10-15 mins on average.
good luck,
drew