Checkmytrip doesn't recognise the US booking, it does recognise the BA reference but then states:
4649 - This trip cannot be found. It may have been cancelled. (4649)
On the US Airways website I can bring up the booking using their reference code and all flights show as active (it also tells me the BA booking reference).
Since all 4 segments are BA codeshare (on BA metal) I cannot do anything on the US website (like select a seat) without going via the BA website and the BA confirmation doesn't appear valid.