To power a CPU fan or a computer case fan you need a 12V DC
powersupply. These can be purchased at radio shack for a few $. Or purchase a fan that is 120V AC
and then this can be wired to a standard two or three prong outlet and then plugged into a regular wall outlet.
To power multiple fans from a single plug you need to know the amperage pull per fan and then match a powersupply that will have enough amperage to provide the juice for the four fans collectivly.
An example lets say a 90MM computer case fan pulls 12V DC
and .02amps. Four fans of this type would pull 12V DC
and .08amps so you need to find a power brick that will do 8miliamps at 12V DC
I personally just purchased an ice cap 120MM fan with themostat built in. Its a little more costly than a DIY
job but a ton easier.