Just a note on a different approach. I was not worried about capturing video in real-time, though.
I have four cameras, so far, that I run into a used 9-channel quad mux which I picked up for $100. The output of the mux is fed into an inexpensive ($70) USB2 video dongle. The mux has a serial port that can control the various mux functions (2x2/3x3 cam images at once, selecting one in particular, freezing the image, etc.).
Example: When someone rings my doorbell, I switch the mux output to the proper channel, capture the image, then switch back to the 2x2 setup.