pin slot4 pin fans can run in 3 pin fan slots and will only go in one way since the header is keyed but will only run via voltage control. Most fans don't mind but have slhtly less range ofUsing the pin and slot, you can put a really tht tolerance on the pin and it then put it in a press fit on one side, and a tht slip fit on the other. This will position it well, and the slot will make sure