You shouldn't have any issues getting into TBIT, if asked just say you're going in for the better shopping (not much in T4).
To exit basically same way as you came in at TBIT, there is a path along the wall next to the security lines.
Then exit the front of TBIT turn right, and into T4.

Downside is having to go through security twice.

Under the rules you and sister should get access to the FL (I would even say the same if you were on a domestic AA flight). Whether it is worth the effort however is another matter.