Getting an unlocked iPhone 4/4S to work on T-Mobile
Will the iphone get 4g speeds


No it won't. iPhones, even 4 and 4S, are not 4G devices. The 4 is for the iPhone fourth generation, but they are in reality 3G devices. So you won't get better speed than 3G (for sure).

I'm not sure for the network itself since I'm in Canada and I don't look at US providers very often. From memory T-Mobile has a different (AWS) network than AT&amp;T so that could also affect your connectivity and speed. You better check the phone versus provider specifications (or someone else doing the same as you).