You are asking on webcamxp. I strongly recommend you to use Netcam Studio also from Moonware. NCS is the next generation of webcam and is optimized for IP cameras. If you are using webcams stay with webcam XP/7.
For 4 cameras at 720p I would use a rather new at least dual core/ 4 threads i5 processor with win10 and 4 GB RAM. Assuming that you do not use the computer for something else. A better choice is a quad core processor / 8 threads. The quad core processor can be a couple of years old. The process of decoding and encoding video signals works best with many cores/threads. The running frequency of the processor is not that important. A HDD is fine, but better is an SSD.
Search in this forum what people are using.