My old thermostat says Berko Model M7
18amp 277vac
22amps, 125,250vac
Single line break
I replaced it with a 4 wire and capped 2 off but even on low the heat is cranking out help please

Sounds like the thermostat you installed is intended to control both lines, and cannot offer adequate resistance when controlling only one. If wiring both lines isn't an option (due to available wires at the point of installation), you will need to find one intended for single-line break applications.