Question : Problem: new motherboard doesn't fit with old power supply??

I have a problem: I don't think my old
power supply works with the my new motherboard:
motherboard: Asus P5KC
Power supply: Mutant Mod reactor ATXPOW480BLU

What I mean by not compatible is that the P8P9 connector (I don't know
if it's still called like that, but it used to be), not the ATX, but
the other one, has 20 connectors on the power supply and 24 on the
motherboard. I can plug it right, but there are 4 connectors with
nothing in it on the Motherboard. How do I know if it's safe?

Answer : Problem: new motherboard doesn't fit with old power supply??

Sometimes you have the 20 pin and then an extra 4 pin power connector for the motherboard.
Its to give extra power to the board.

Normally you should be able to get the system running without the extra 4 pin power connector, the extra power connector is for system stability when you have a lot of load on the board itself.

Just be sure that you didn't plug the 20 pin cable in wrong (normally you can't do it wrong and can only put that there and the 4 pin one in its bit but you never know)