Quote Originally Posted by Ughmahedhurtz
Using wireless keyboards that allow binding one keyboard to all 5 machines is inexpensive and definitely works well (I've been 4-boxing with 'em for ~1.5 years now). You can get decent keyboards for about $20 each, so $120 total instead of the $400 figure I seem to recall. Not as cheap as software multiplexers but gives you hardware reliability/latency and has zero issues with extended keybinds and even works with stuff like multimedia keys so you can mute all 5 boxes at once.

The only downside is that some of these tend to be very cheaply made and/or atrocious quality control, so you may have to take a couple back and exchange 'em to get working ones.
I went down this route at first only to find the keyboards (Logitech Cordless Internet Pro) were not gaming safe, as in press several keys at once and after a few are held down new key presses are not registered. It would be worth going slightly more expensive (these were £20 each new) to get keyboards that will work well in games.

In the end I sold one keyboard to my dad and use multibox and synergy along side a Belkin n52 which performs better, is easier to set up and costs nothing.