If the batteries were fully drained I can see 6 hours at 375mA (375 * 6 = 2,250mAh) and not all of that is going into the batteries. It sounds like a lot was going into heating the charger 
Could be the voltage showing on the D4 is way off. Perhaps you can check it against a DMM when you next charge fully drained batteries.
The batteries are probably hot due to the charger being hot. Is that normal for D4’s?