Most 24pin power supples use to be used for servers and now are more common for the Intel core solo/duo and the AMD 64. The 4 pins arn't needed. Just disconnect them and use a dremel and cut the connector down to size if you have too. Pins 21,22,23 are +5v and pin 24 is ground. You might beable to still connect it the way it is and the 4 extra pins just hang.

Now if it's the kinda that has 20pins and another cable with 4 pins it's just used for older P3 and P4 systems. Just extra two 12 volt and two extra ground leads. Same as above, don't need them.

Just follow the standard mod for it. The extra leads will come in handy for fans and such.

heres a 24pin pinout

Another link that might be helpful.
To live is to let die