We did both the four night and the three night, and they had very different vibes. The three night was more of a weekend, "party" crowd (I'm using this term loosely--Disney concierge never gets crazy--just more people hanging out and getting drinks in the evenings), while the four night was more subdued. The busiest the lounge got was from around 5-6:00pm each day.
The sundeck was never crowded when we were on. You won't have to wake up at 5 am to secure a chair, as people aren't rude about keeping their stuff and claiming chairs on the sundeck for hours. I've never seen that in concierge on Disney. There's so much to do on the ship that I didn't see people spending a lot of time up there anyway, plus it is huge with tons of seating.
I don't know about turning the loungers--they are in there pretty tightly, and they are pretty heavy duty and high quality, so likely weigh a lot.
