change carriers
if i buy an at&t galaxy s4phone can i use my current t-mobile sim card to make it work.also is at&t connected with t-mobile in any way.
No you cannot. T-Mobile and AT&T are two completely different services. AT&T's SIM cards are shaped differently and a T-Mobile SIM will not fit into the AT&T Galaxy phones. I do suggest switching completely over to AT&T! I have found that their service is generally better and they seem to have better coverage in many areas!