Geez dude you are really desperate to cut costs whereever you can't lol I guess I can't blame you.
If you use apptousb on PS4 you can keep a hard drive formatted in Exfat format. The problem is PS3 and Xbox 360 use Fat32 and while PS4 can use Fat32 it's not practical because you can only put games that are 4 gb or less on it most games on PS4 are way bigger. So it wouldn't be ideal or very good to try to do this unless you use apptousb and want to also hook the hard drive up to the PC on occasion.
Actually that's true if you make different partitions you could use them on all those consoles as well. I'm not sure why not just have one for each console so you're not switching back and forth all the time I would hate that personally.