Iphone 4/4s on Tmobile network?

I want to purchase a Iphone 4 or 4s and use it on my Tmobile network. Seeing as many people sell their used iPhones I wanted to see which ones are compatible to my network. I. E. Tmobile uses sim cards. I know that I would have to switch to a micro sim card when I switch it to Tmobile but I'm concerned as to which companies (Verizon, AT&T, Sprint) even use a sim card or have a sim card slot for me to be able to use the phone on Tmobile network.

Added (1). Oh and if you know of anyone selling an Iphone 4 or 4s for cheap let me know.

First off, make sure to get an Unlocked iPhone that has a SIM try, else it will not work on TMobile. Verizon and Sprint versions of the iPhone do not have a SIM try that you can put a TMobile sim car in.

AT&T still uses micrSIMs as well as SimpleMobile as far as my experiences go.

When you use an iPhone on the TMobile network, they restrict you to EDGE network speed, which I can tell you from personal experience is painful to use. Everything takes FOREVER to load, forget about watching anything on youtube, even at 240p. SimpleMobile sucks just as bad in terms of their data speeds.

I eventually got fed up and went back to AT&T and been happy with their speeds despite their robbery prices. I don't mean to deter you from your dreams of being on the tmobile network, I've just gone that path and regretted spending a hundred bucks switching all over the place.