I would start with backing up the phone, then performing a full restore of the iOS. It could be corrupted software and this is, unfortunately, the least invasion step.

If the overheating continues with just the iOS, inspect the dock connector and headphone jack liquid immersion sensors - if they are red/pink, then moisture has gotten into the unit. Use a flashlight to aid visualizing the sensors. In my experience, it takes _a lot_ to set them off in the iPhone4 - I had a coworker drop her's in the toilet, she fished it our immediately and got it to me fast. The sensors were still white, inside and out, then phone continued to function just fine after I disassembled and dried it out.

If you can, don't restore your data from the backup yet - it is possible there is corruption in the data and leaving it off for a few hours can help with issue isolation. If the phone appears to be working properly with just iOS, then restore from backup (right-click on the iphone in the left column of iTunes for this option). If the issue returns with the data from your backup, restore iOS again but not your data backup.

If all else fails, and the liquid immersion detectors are white, it's time for a trip to the Genius Bar to get it swapped.