It does look like IB8721 arrive to T4S, so 50 min should be enough to transit through T4S.

Last week coming on a LHR-MAD which arrived at T4, IB disembarked through the back door all connecting passengers to T4S and brought them there by bus. We had about 20 min delay, that might have been the reason. So I assume that if you have any slight delay they would make sure you make it to your next flight.