power supply problem

I have a xion 450 watt power supply that has been used on a motherboard using only the 20 pin power connector.
I have replaced the motherboard with one that requires the 24 pin connector.
When I connect both the 20 pin and 4 pin connectors (and connected correctly), nothing will power on when the machine is turned on.
I have connected a different power supply that has a 24 pin connector and the machine starts perfectly.
I know this is a stupid question, but since I have been using only the 20 pin connector, is there something special I need to do the activate the other 4 pin connector?
Thanks so much for you help.

 

No, you don't need to "activate" the other 4 pins.

Your post has confused me...

Does your motherboard have a 20/24 pin connection?

because i dont think you should have been using a 20 pin connector on a mobo with a 24 pin connection!

Burrell

 

Thanks for the responses.

The old motherboard was a 20 pin.
The new motherboard is a 24 pin.
The power supply was working just fine with the 20 pin, but I get nothing when I hook up the additional 4 pins to the 24 pin motherboard.
The power supply that was being used is XION 500 watt.
The power supply that I stuck in, that I will not leave, is a Thermaltake 1200 watt.
The old motherboard was an ASUS P4P800 SE (20 pin)
The new motherboard is an ASUS M4A785-M (24 pin)

It's my brothers computer, and the motherboard was broken when he
went to remove the graphics card.

The Thermaltake power supply is one that I have here for a future build, and considering the difference in price, no way am I letting my brother have the Thermaltake.