Hi Nils and welcome to the forum!
Excellent question since you maybe need to rethink your project depending on what you want to achieve.
For system requirements I would say this:
- I would recommend an Intel i5 processor. It might work with an i3, but with 4 cams and 20 fps and decoding to mp4 it is a lot of work for the cpu. If you want to increase resolution above 640x480 go with an i5.
- memory is not that critical so 4 GB is fine.
- storage depends on the time of the recordings. A regular mechanical HD is no problems. An SSD is of cause best, but cost a lot and not necessary for 4 cams.
- As operating system Windows 7 or 10 or a Server version.
Most important is actually this!
You want to have 4 usb cams with NCS. NCS do not handle USB cams that well due to the programming language and the USB bus. The problem occurs when you have many identical USB cams from the same manufacturer and the same model. It is a problem with the identification of each cam. One cam works perfectly and maybe two depending on the usb bus in the computer, but more than that will cause a problem. Several user in this forum have had this problem. The solution for them have been to buy a separate USB card and mount that in a free slot. Another solution is to use WebcamXP/7. This software focus on webcams and where this problem do not occur.
If you already have the usb cams just download NCS and WebcamXP and test.
So a Tablet PC will not work. At least a Laptop. Depending on your budget go for a laptop that is a year or two old. That will probably work well.