I've used the new MAD several times and it was fine, there shouldn't be a problem. I think BA is switching to T4 from T4S due to pax complaints at how long it takes to get out of the terminal. They were in T4S originally (idea being it was the non-schengen terminal). That doesn't really change anything (5 min train ride between T4S and T4), if they have made that change. MAD is claiming the lowest rate of mishandled luggage of any airport in Europe, fwiw, since the new T4 opened. Two hours is more than enough for interlined luggage, which is of course possible on BA with separate tickets as long as it is another OW airline, and it is.