Rule it out by making a usb live linux disc, then boot from it, and test each port. If nothing works at all with all four ports at the rear, check the setup utility/bios after rebooting to see that they didn't just mess around and disable any of them (my friend did it to me before as a joke). Load setup defaults, save and exit. If they work, there's your problem. If not, it's hardware, and usb cards aren't that easy to come across, though, it's a bit pricy if you do have a free slot!