I think weight is going to be your biggest problem. I don't know that any of the typicaly swivel mounts will be able to hold 4 monitors.
Dell's site says the 2001FP weighs 18.72 pounds with the stand. I'd guess that they weight about 16 without the actual stand. So figure for four monitors, 64 pounds.
This page specifically mentions supporting up to four monitors with their mount. I'd give them a call and ask them if they have anything which will fit your needs.