Could be one of 2 things I think.
1. Your switch is not making the proper contact.
2. Your fan motor is pulling more current than it should.
I would start with a trip to the scrappy for another switch - the cheapest option. Also - I don't know where the fan resistor is on the Golf. If it is in the switch then running position 4 will bypass the resistance and it should run cool. If it runs cool, it points to the switch. If it runs hot it points to the fan. This only works if the resistor is in the switch though.
I guess a blocked pollen filter could cause the fan to struggle. Try pulling that out and see if it improves things.