I have never done this before but in a couple of days we will be staying at the LM Barcelona. Have advised them there will be 3 of us and they will charge us for the extra bed. Now the hotel is sold out and have no more rooms but my 4th friend is coming along. I don't think they will allow 4 in a room?

Basically we get there at 7pm put our bags down and don't get back till 4am then leave at 10am for the airport. So my question is shall I advise them now there are 4 of us and get refused or wait until check-in and then advise them hoping they won't have any option to say no or just sneak the 4th in at night?