I would say a quad room would sleep 4 and a double is a double bed only sleeping 2

I agree with you, Lynwestie, BUT ..... sometimes a double room can mean a room sleeping 2, one in a bed ie a twin. This can happen in Spain, Italy, Croatia and Turkey (in my personal experience) so if you want a room containing ONE BED to sleep TWO PEOPLE you must make this very clear from the outset!!

There may be a special phrase needed eg camera c o n letto matrimoniale in Italian or un grand lit in French or even ein franzosisches Bett (a French bed!!) in German.

I'm sure other members can add extra useful phrases in other languages!