Fans won't stop your PC from booting. a lot of fans use 3-pins instead of 4-pins, so don't worry about that.. As long as you can see the fan run when the pc will boot.
If you tested all of your components (CPU, GPU, RAM and PSU). It must be your Motherboard. You should go back to the shop, and tell them they're both broken. If they say they aren't, they should be able to show you. If they show them all working, it must be one of your other components.
But troubleshoot your motherboard first!
btw, did you attach the motherboard correctly, and didn't you forget the standoffs!