My memory is a bit fuzzy, but I'm pretty sure that at T4S you can't just get off the plane and join the queue... you are directed straight upstairs to go through passport control and/or security control on the upper level. T4 you just get off the plane and there is a passport check right there so in theory you could just go to the back of the queue. (although an IB metal flight is much less likely to be same plane turnaround)

Query whether you really save all that much money after paying for the LHR-MAD (no cheap one ways) and the risk of getting it wrong, even if you build in some extra hours in MAD (although the lounge is fine for a silver card holder).