In Illinois the law is any trailer with a GVW of 3000 pounds or more requires brakes, dual axle trailers of between 3K and 5K need only one axle with brakes and over5K all axles need brakes. Your vehicle sticker will show the GVW since most states base their licensing fees on GVW, they also require a brake away brake capability
Now if it has 4 pins, that is just for the lights, you have a ground, right, left and both (running lights....headlights on)
To be safe I would not want to tow a trailer of over 2K without brakes since a trailer will easily jack knife you in a split second
To have electric brakes I believe the minimum would be 5, the fifth wire would be for brakes, you don't see that much but the standard is a 7 pin which includes a positive for recharging the battery on the trailer
check out this site for detailed wiring info
Trailer Wiring Diagrams | etrailer.com