This ought to be interesting. I'm on Flexpay with Tmobile, with the "Everything More Plus" plan, with unlimited voice minutes, unlimited texts (which I don't use), and supposedly, "unlimited" data. I originally went to this plan, because I was on a 1000-minute plan, and I was exceeding that monthly (at a cost of $0.20/min excess).
Except the price of my plan is $79.99, but they gave me a $10 discount on the data part of the plan, because I have an older non-smartphone (but it can still tether via USB, which I may try some day.) It's not a 3G phone, nevermind 4G.
I was told by a TMobile rep at a corporate store, that they have discontinued FlexPay plans, so that if I were to switch to pre-paid, I couldn't switch back.