Another Dodge site reads differently, i did the BY VEHICLE part, you have to fill in the Year, the Vehicle, Bed Size, & Engine
2009 DODGE RAM_1500 1500 REG ST, REGULAR CAB, 2WD, SWB Bed, 4-Spd. Automatic VLP 42RLE Trans, 3.7L V6 Engine:
With 3.92 Axle Ratio Axle Ratio You Can Tow 3850 lbs http://www.dodge.com/towing/D/vehicle_to_weight.jsp
I just purchased a pickup after towing my 5800 lb travel trailer with a Jeep GCherokee with the 5.7L Hemi. What ever your towing capacity is and yours is around 4000 lb., you can still tow a trailer on level ground but woe be when you hit even a slight grade. My Jeep was rated at 7200 lb towing capacity but I found (and can't remember where I found it) a web site that shows what the trailer weighs when you start going up a hill. For every degree of angle your trailer gains a lot. My 5600 lb trailer weighed about 8500 lbs on a hill with a 6% grade which went way beyond my max capacity. Again, the trailer can be towed with a small engine car so long as there are no hills and the tongue weight doesn't bring the front wheels off the ground. Its the hill and added heat that will kill the motor and transmission.
Also keep in mind your tow rating is reduced by any passengers weight, drivers weight over 150 lbs., cargo and accessories. it is also good idea to allow a cushion rather than bumping up against the tow rating, especially if hills and mountains are involved.