My wife and I are on a multi-country tour. On the return back to the United States, we're flying DXB-LHR-BOS. First leg is Gulf Air and second leg is United Airlines. I understand I have to transit from Terminal 4 to Terminal 2 and can do this airside, but various sources are rather confusing on these two questions:

• Does my wife need a transit visa? She's a Ukrainian with a US green card. From what I've read, as long as she stays airside and we depart on the same day then we don't need one. Can anyone confirm this?
• Do we need to recheck bags? The flights are under the same reservation, but I don't know if Gulf Air and United Airlines have an interline agreement (or if it even matters).

Thanks in advance!