Hello a double is 2 people and the other is 4 people to a room.
I ghink double room is the rate is for 2 people in 1 room. A quad rate means 4 people in 1 room (usually 2 fulls or 2 queens).
A double room would be for 2 people, I would guess. I don't know what a quad room would be. Contact the hotel directly.
Get an answer from Hilton Garden Inn Bowling Green staff and past guests.