COD4 only recognises the following on a mouse;
left button
right button
scroll up (scroll button if it has one)
scroll down (scroll button if it has one)

Any other buttons can't be mapped in game.

However, your mouse software may be able to map a button to a key. For example, map G to side button. So you can then use the side button or keyboard G to throw grenade.

I don't know if this works for Habu, it does for logitec using the 'setpoint' software.

It may be worth members adding there keyboard / mouse configurations to help. I only use scroll up and down on the mouse. Everything else is done well on the keyboard. I'll post mine after this reply.