I dug a bit into the various plans tonight. Verizon and Sprint cost $160 for four phones. T-Mobile is $140 with their current discount.
Sprint and T-Mobile (via the new T-MobileONE plan) offer unlimited data but they restrict video streaming to 480p. T-mobile limits tethering to 512 Kb (3G speeds) unless you add an additional cost tethering option to your plan, throttles you once you exceed 26 GB of data, and charges $5 a month if you don't set up auto payment.
Sprint limits tethering to 5GB a month, after that it's $15 per GB.
Verizon allows you to roll over unused data to the next month.
All told, Verizon seems to be the best deal out there, a bit more than T-mobile but the network is better and there are less restrictions.