USB ports are supposed to be able to supply like 500 mA or something (not sure if that's exactly the number but there's a standard limit) but if you have a hub that's not powered, all four devices draw from one port that can supply a max. of 500 mA. So if you have gamepads with vibration feedback, those probably draw a good deal of power... if they draw 200 mA each, then you can plug in two for 400 mA total, but plugging the third would be 600 mA and the port can't supply that. Just a bank of 4 or 5 bright LEDs would bring you close to or above the 100 mA mark, so it's pretty easy to break the 500 mA limit with four devices.
If you power the hub, then each port on the hub can supply 500 mA again.
Looked at my hardware - my Microsoft optical mouse has '100 mA' marked on it. Same with my keyboard. My graphics tablet says 40mA, an old Logitech joystick (analog) says 25mA. So I could plug all four of those in with no problems. 74XX chips and lots of off-the-shelf logic components (like the IPAC has on it) often draw more power than optimized commercial products.