I use a Matrox Parhelia for 3 monitors, but would really like 4, I'm bidding on a Matrox G200 MMS Quad, I used one for a couple of years @ work and it was very stable - more than my Parhelia on XP, so i''ll be sticking with Matrox.
There pretty cheap on ebay, normally go for around 70GBP on...