I haven't tried it, but if you try it, I think your best bet would be to use non-HD webcams (preferably 640x480). Any resolution larger than that will significantly increase the CPU % required because the images will be to be resized before being sent to iCam. I use 4 IP cameras with my system, and have no problems, especially since the images are 640x480 and I have the frame rate set to 5fps. Another downside to webcams is that you can't change the fps.
I know it doesn't answer your question, but hopefully it will help guide you in case nobody else responds. If you build a new computer and it can't handle 4 USB cameras, you can look into IP cameras.