I have done post processing at 30fps with 5 ps3 eyes in OpenCV on a core i5 750. This is a quad core machine if you aren’t familer with the i5 750. Just some basic blurring and edge detection use around 60-75% of the CPU. I don’t know how CCV’s performance compares but that should give you a decent idea. I have also written my own multi-touch detection with all 5 and have gotten it working fine at 60fps with all cameras. Sorry but I can’t share the code but it wasn’t particularly difficult so you shouldn’t have much problem.
If you are going to do a lot with the data I would recommend using CUDA or OpenCL on the GPU. I only use about 7% CPU when I run my code on GPU for 5 cameras.
Also if you are going to use 4 cameras you will probably need a full speed USB 3.0 system with 4 ports or they wont run at full speed.