We had a cabin on Deck 4, although more midships than yours. We found it quiet and comfortable, although it did have the hull balcony. This didn't really matter because we were in Norway and it was far too cold for balconies!
The seas were like a millpond the whole time, and we had no sense of motion at all, but you cabin is quite far back and there may be additional movement if the seas are rough. I never noticed any engine noise on the ship, even in the Britannia Restaurant which is much lower than your cabin, so hopefully that won't cause a problem.
Interestingly, we've booked a deck 8 cabin for our next cruise (to the Med) where we hope to benefit from the sun.