Which engine does the MH have? GM 454? MOPAR 440? Ford 460?
The 4-wire trailer plug means your MH is probably not wired to have a trailer brake controller. The brake controller requires at least a 5-wire plug, and more common is the 6-wire horse trailer plug or the 7-wire RV plug.
You need to add a 7-wire RV plug to the back of the MH. Even if the trailer you want to tow has a 6-wire round plug, I'd still add the 7-wire RV plug and then add an adapter to plug into the 6-wire plug. Most trailers with brakes now use the RV plug - even new horse trailers. Older horse trailers (or livestock trailers) may have the 6-wire plug.
The 4-wire plug is only for trailer lights. It has left and right turn signals, tail lights, and ground wire. The turn signal wires are also used for brake lights. The tail light wire is also used for running lights.
The 7-wire RV plug adds wires for trailer brakes, battery charger for the battery in the trailer, and another wire that's usually not connected to anything, but identified as the backup light wire or auxiliary power wire.
Trailer Wiring Diagrams | etrailer.com
How to add a brake controller to connect with that brake wire in the 7-wire plug depends on which engine you have. Probably the easiest is to buy a kit that includes what you need, with instructions of how to install it. Such as this one:
Universal Installation Kit forTrailer Brake Controller - 7-Way RV and 4-Way Flat - 10 Gauge Wires etrailer Accessories and Parts ETBC7
Scroll down in that link and you'll see lots of detail about that kit.
Instead of a universal kit, even better would be a brand-specific kit to add a brake controller and 7-wire RV plug to your MH. I know the ones for a Ford chassis are available, but I'm not familiar with other brands.