Frame grounds are famous (notorious) for unreliability.
Our 2000 Scamp
16 ft has a hard wired ground to all lamps and fixtures.
The 7 pin connector has one pin for the ground connection to the car.
Your 4 pin connector works only for tail, stop and turn signals, with the side marker lamps also connected to the tail light
If you have a battery
on the trailer, it would need a 5th pin with a hot lead from your car battery
(and a fuse under the hood)
If you have electric brakes
, a sixth pin is needed, and backup lights a seventh.