I have never seen an adapter that allows a four pin to go into a seven pin pig tail. Go to u-haul and buy a set of tow lights that plug directly into the OME four wire plug you have. Or better yet have them install a seven pin on your vehicle and tow the trailer correctly.
BTW the Feds say anything at or over 2000# needs brakes with a brake-away cable and battery. Will you get caught running with out them, most likely not. Unless you are running without lights, then you are waving a very large red flag that every LEO will chase after.
Don