We were on Deck 6. The aft-wrap cabins are the only ones we book, so we're quite used to them. Yes, you do feel motion back there. We did have one night out in the North Sea where it got pretty rocky, but for the rest of the time, no bother at all. Many times it's also noisy/vibrating back there when pulling into ports while the ship is docking. But, for this particular cruise, that wasn't an issue at all except for when we were docking back at Rotterdam to end the cruise.
Deck 4 has a super wide side section but don't forget it also has the wiper contraption that kind of just is "in the way". It's also not covered, so if raining, not good.