As far as I know it's your keyboard. My old one supported 4 simultaneous keys while my new one handles 3. I hear others supporting at least 7.
For instance, here's a keyboard that can support 7 simultaneous keys at once.
http://www.ideazon.com/global/products/ ... yboard.asp
"Registers up to 7 simultaneous key-strokes, in comparison to 2-4 on most other keyboards! Jump, move forward, lean left, change weapons and shoot at the same time!"
Edit: Misread your post. I don't know if there is a workaround, I guess it's a built-in limit for your keyboard.