first, check to see if the laptop already has a VGA port. if it does, then you could attach one monitor to it. if he wants 3 other separate monitors, you could try the usb adapter that you mentioned. in theory, you should be able to use multiple ones.
you might check and see if your client would prefer to have just 1 monitor. these days, you can get a really big monitor (22" and larger) and it's not that expensive. i don't really see the point of having 4 separate monitors when you could just have 1 big monitor attached to the VGA port that is usually a standard feature on a laptop. i bet the overall cost would be about the same, or possibly less.... your client could then put up the 4 windows for the 4 cameras on one large screen....
his desk would have a lot more space on it, too...