Go look on eBay and get a low voltage alarm for a 4S. It attaches directly to the balance connector. Mount it so you can see it and it will alert you as you approach low voltage. Exmaple of whats available on eBay -

http://cgi.ebay.com/New-Low-Voltage-...item335f5b729c


Looks l;ike the Chinese vendors have taken this over.....