I'm glad this topic came up because I've also been asked to setup four wireless security cameras.

My thought would be to use a raspberry pi 2 (or 3!) and have it connected via USB to a external 250GB SSD then to use zone minder to capture the video.

The questions I have are:

1. Knowing the USB's all share a single interrupt... will the pi have enough throughput to capture and storage the video?

2. How many cameras can a pi reasonably support?

3. Does zone minder have retention policies?

Thanks,

Tim.