Is there a possibility to check out at 3-4 am?
Basicly, while checking out you only put your keys in the box, doesnt matter what time it is.
I think yes. You just leave. I left 7 30 when nobody was at the reception yet. The street is safe outside too.
Get an answer from Diagonal House staff and past guests.
3 Days in Barcelona
Barcelona for free
Barcelona Off the Beaten Path