If you have a midday-ish arrival and a 21:30 flight then I think you have much longer than 4 hours to fill. That seems like a lot to me, however nice the waiting room is but others find ways to make the time fly. I'd say take lots of entertainment with you, enjoy a leisurely meal (but don't forget you'll be eating on board too!) and just relax.

Don't bother with a cabana booking - they're nowhere near as nice as they sound and unless you really need to sleep, or privacy for some reason you're better off enjoying the experience at the bar, or on the terrace etc.