So far as I know, only Verizon has LTE right now (true 4G); what T-Mobile and AT&T are calling 4G is their HSPA+ network, which itself is pretty darn good but is squarely outclassed by LTE. Both T-Mobile and AT&T are planning to have actual LTE networks set up in 2013, but it will probably be 2014 before they can boast coverage that can compete with Verizon's LTE network.
However, once T-Mo and AT&T have LTE set up, they both have an HSPA+ network their customers can fall back on. Verizon, on the other hand...
(I do not know how Sprint factors into this. Never had them myself and, based on T-Mobile's up-and-comingness and broader network, I probably never will.)