Ah, yes. my oversight.
IMHO, a half ton Ram with a Hemi and a 3.21 axle (designed for fuel economy) should be fine pulling a 4,200 lb. trailer. This would be for flat terrain or low hills, and not trying to win any races. But, if you are planning any trips to the mountains or 100+ degree traveling then you're asking for problems with tranny temp. And, if you're going up to 7,500 lbs. then you're in a whole new ballpark. HD half ton, with all the tow support items would be required as a minimum. Me, I would take it to the next step and get a 2500. It's much more suited for pulling: heavier frame, bigger brakes, suspension, drive train - you name it. The last thing you want is some sort of breakdown and a ruined trip because you are pushing a light duty vehicle instead of letting the added beef of the HD truck take care of business. My Ram 2500 diesel splits time between pulling a Jayco 5ver and my 24' car hauler with camping gear and my vintage Jeep. Both run about 8,000 lbs and I would NEVER try pulling either with a "halfie." Choose carefully.