are you buy these off of ebay , provide a link ?
What is the price ?
I recommend at least 8 GB of memory for a windows 10 PC running BI. Make sure there is space to mount at least 120 GB SSD for the operating system and the BI program. Depending on the amount of time to keep the video a larger WD purple drive may be needed. But for a start the 500 GB drive should provide a good starting point for storage.
The number of cameras is not the issue, it is the MP/SEC. 4 cameras at 1080P at 10 frames a second, is completely different than 4 4K cameras at 20 fps.