The only reason to have one player is if you're going to have content stretched across all 4 monitors. You could do it with three players but syncing can be tricky but not impossible. I have not tried it, so I'm not sure if it is possible to utilize multiple video cards with the player. It's very hard to give advice on setup as there are more variables that play rather than just a simple answer. When I give my answer, I assume the user has the screens right next to the players and there is nothing in between. It's important to note this because your requirements may need multiple appliances that may require something completely different than what I recommend. Idealy, having one player per screen would be best as you would have at least one redundant system, meaning if one player goes down, you could piggyback off another player until you get the other one fixed.
Now if you have a rather large budget...I would try the Matrox C680 card in a workstation that you build. This card can push out content to up to 6 monitors. They have a 4 monitor unit, but it's less than the current res specs of MS to get you the 4 screens. So for this card, it will run you about $650. Matrox has been the industry leader for video wall systems. The CPU doesn't need to be a beast, but you need to make sure you have a power unit stong enough to run the card with the rest of the system.
