4 min limitation, of my memory serves, is because reverse thrust blocks off the cooling air flowing over the engine mounted accessories. The limitation is then to stop them cooking up.
Regarding an earlier question, you really need a flight crew member to answer, but looking at the RHS panel on the Concorde Heritage site it looks to me that the symbol is the landing gear indication - nose, two mains and a tailwheel. Haven't a clue what the other indication might be.