That's essentially a "video wall" and there are probably hardware solutions for that. Dataton WatchOut is a software solution to this which runs on Macs and PC's. But why would you want to go this route? You will always see lines where the images meet. If you're just trying to get a brighter image then get a brighter projector or overlay more than one. 4 projectors are going to be awkward and expensive, plus I don't think the results will be very good.

Folsom Research makes a device for blending two or more projectors together. It is absolutely seamless. I have a critical eye, and I thought the guy was lying when he said it was two projectors side by side....until he shunted one pj and only one side of the screen had an image!