It sounds like the heater is stuck on. That could be a damaged part of the climate control system in the dash, a stuck heater valve, or similar. The heater core is in the cabin, so if it is stuck on "hot" you have to deal with it until it is fixed.

By the way, the 3-4 minutes is the time it takes for the engine coolant to warm up and bring hot coolant to the heater core.