If 4g is available, you use that, if not, you get 3g (all with no roaming).
And yes, you do have 3g, I have no idea where the rumor came from that there is no 3g. When you exceed the data quota you drop to 2g, which I hear is about 100k.
Be sure to check coverage with neighbors and such before you switch. At my house I get 500k-1.8meg on Virgin in my house, but my neighbor across the street on T-mobile gets spotty data outside the house and no data inside. In another neighborhood, you get no data on any carrier, but you get can get a cell signal with T-mobile.