buy licenses... if you wanna be legal there isn't really an option. Unfortuantly 4 licenses are a to smaller number for MS to really worry about with their bulk licensing programs (I might be wrong, although they are a oftan a bitch to conform to as some cases you are "renting" the software) also you may be able to get an OEM version which is cheaper (OEM just requires that you buy a piece of hardware, sometimes this can be simply a mouse pad)