ISboxxer with innerspace
ISboxxer is free, innerspace requires a monthly/yearly fee (it's not terribly expensive)
Isboxxer just makes the screen set up better (like you can have all 4 toons open in the screen without having to tab)
Innerspace allows keystrokes to be cloned in all your windows. (So if you press 1, 1 is pressed on all your open boxes)
If your boxing 4 or more, innerspace is well worth it. It makes boxing amazingly efficient.