I do not think its at all good for a boat to be locked in 3-4 feet of ice. However, I believe that the damage comes when the ice starts to move away as it breaks-up/thaws. Locked in ice is not good for the boat but will most likely not cause the real damage (unless seacocks freeze and burst). Docks and poles typically get damaged when the ice is pulled away. Now every situation is going to be different here so I am commenting based off general experience.
I use an ice-eater for my 30 footer in a salt-water creek in NY. Low-low tide the keel is at rest on very soft mud at my dock (~4 feet directly under the boat and maybe 6 foot several more feet away from the dock). The ice-eater works fine and no ice ever develops under these circumstances. It could be that its angled in a way to pull water from the center of the creek where its deeper and slightly warmer but again, no ice forms around my boat.
Every situation is going to be different but the above is my experience in cold-cold 20 degree winters.