I have an iPhone 4 locked to Orange, will a t mobile sim work in it?

Thanks

No, and t-mobile only supports the iPhone 5, so you couldn't bring yours on to their network.

I don't know

Yes, they are both now a part of EE so the sim should work fine.

You could take it to your town centre, or their are websites that can do it. It is possible to have your phone unlocked and it shouldn't cost to much. You could also download some software that would allow you to do this. This website may be of some help http://giffgaff.com/unlock/apple-iphone-phone