problem with multiple devices using USB ports

i'm using 4 devices right now.The devices are all rfid readers that have usb connection.
my comp have 4 usb ports.

The problem is, when i put all the 4 devices into usb ports, my comp can only detect 2 of the devices.

Now, i tried with another PC of my rommates. (My rommate also have 4 usb ports). And his comp also detect only 2 devices of 4. I tried with other rommates, which only give the same result.

Then, i tried to install an usb hub (PCI card that have more 4 usb ports, so total usb ports become 8 in my comp), and the problem is just the same, my comp only detect at most 2 devices of 4 devices.

What's wrong with all of these ? Is it true that our computer "JUST" can use 2 usb devices at a time ? Is there any thing that i missed ? Configuration perhaps ?

I'm using Windows XP Sp2, always upadated. My rommates also.

Thx for reply