You're going to run into three different problems.
1. power - you'll need to make sure that each camera has adequate power. Running two or more off even a powered hub could be problematic. This is the least of your problems.
2. CPU power - encoding full HD video is a VERY CPU-intensive activity. Especially at full 24fps (or greater) capture rates. I have a quad-core Ivy Bridge i5 at 3.3ghz and it runs better than 60% CPU when encoding 2 webcams at full HD. My guess is you'll max out your CPU trying to do three or four. But that's not your biggest problem.
3. Bandwidth - The USB 2.0 specification supports 480mb of bandwidth for the ENTIRE USB bus. That means that all the devices on your system share that bandwidth. A single HD webcam can easily eat up 50% or more of that bandwidth. With two, you're probably going to be dropping frames, and as you have seen, you can't even run 3.
The short answer - you will most likely never be able to run 4 USB webcams at 1080p 30fps on a single computer. There is simply not enough USB bandwidth (or CPU power) to do it.