http://www.cnet.com/news/t-mobile-undercuts-at-t-on-4-line-10gb-family-plan/
T-Mobile undercuts AT&T on 4-line, 10GB family plan
Good through January 2016. It then goes back down to 1GB per line. Though a lot could happen in the wireless world in 17 months. It might be permanent by that time.
T-Mobile undercuts AT&T on 4-line, 10GB family plan
Customers can sign up for the new deal through September, and the boosted high-speed data lasts all the way until the end of 2015. The company said the promotion is available for both current and new customers.

Good through January 2016. It then goes back down to 1GB per line. Though a lot could happen in the wireless world in 17 months. It might be permanent by that time.