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.