If the car was delivered in the US, #4 fuse supplies the 4-way signal flasher unit, and the two heating coils for the automatic chokes (front & rear). The front choke heater is supplied directly from the fuse, however, the rear heater is supplied thru a relay which is grounded thru a temperature switch. Whenever there is an opportunity to add a complication to the process, Mother Benz is very likely to have availed herself of that opportunity!