You can't with the default conditional formatting, but you can 'trick' it
into letting you use more. Check this out:
http://www.mcgimpsey.com/excel/conditional6.html

"Alison" wrote:

Is there any way to increase from "3" the number of conditional format
arguments allowed?

I have 4 arguments and the cells this applies to will contain one of the 4
arguments or be blank. If the cell is blank, no conditional formatting
should be applied. For each of the 4 arguments, a different cell pattern
colour would be applied.

Thanks!