Rather than having 4 monitors I would suggest having a newer GPU… which graphics card are you using? I don’t think something old enough to not have a single HDMI port would be capable of giving input to 4 displays. If you’re certain you want to try though, motherboards generally will have a port of some kind, if it’s an older mobo probably VGA or DVI but if it’s newer it may have DP, HDMI, perhaps both. Still though, I wouldn’t recommend trying to run all 4 displays if you find a way lol.
I did a quick search and the smallest storage model is 32gb, so the OS shouldn’t be taking up anything close to 90% of your storage. If you have an SD card available I’d suggest that for storage.