I am biased in the fact that I prefer one copy of WoW running per PC. I personally set a goal to 5 box and waited until I had the hardware before I began. If you really want an enjoyable experience, I would do lots of research and planning before beginning. It can actually save you quite a bit of money and frustration. I would either do 3 or 5 rather than 4. A cheap option is 3 computers that are capable of playing WoW, 3 monitors, and wireless keyboards for broadcasting. I am personally using Synergy for my keyboard broadcasting until I upgrade to Vetras.

Really, you need to factor in what exactly you want to do and what your budget is. Come up with goals and plan out how to achieve them. There are plenty of us willing to help you put your goals in action