I just traveled to Madrid with my dad and we both came from different countries, so we needed a meeting point in terminal 4, as he was arriving at T4S. The meeting point we used was the Caffriccio restaurant, it is on arrivals hall(where family and friends wait for their loved ones) at Terminal 4 just on the right side if you are going out or on your left if you are waiting on the hall. My dad had to take the little subway that connects both terminals and he did it just fine, then picked up his luggage and came out to find me. Very easy, no complications he is 75 yo. In the other hand I arrived to T1 and it was very easy to find the shuttle to the other terminals, just walk to the exit all the way following the signs. and when you arrive to T4 just walk to arrivals, following the signs, very easy
NOTE: the only complication I found in this airport was that the elevators go to different levels, so take a look at the signs above the elevators some say -1 but not -2level for example I wanted to go to take the train and the elevators on the left of the airport didnt have the "renfe" logo, were as the ones on the right did. So just pay attention to that. Another one is when you want to go to terminal 4S some elevators don't go to the lowest level(train or people mover) so check the signs above them
Hope it helps!