I came with a group of 5 people on a Saturday night and we had no issues getting a small room, so you shouldn't have any problem there. In terms of age, I saw many people who looked to be teens. They don't seem to exclude underage people from karaoke.
The group of 4 isn't a problem sometimes their rooms get full but usually it's not bad. I'm not sure if their is a age limit. Since they do sell alcohol.
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