400 x 400 is correct. I mounted a 43" on an art. mount. It replaced an existing TV that was semi-built-in to a cabinet. So it will actually sit nearly flush to the wall.
Go with an in-wall mount so it can still sit against the wall, or it will not look much like a picture frame, which I assume they are wanting, hence the Frame TV.
"When I eat, it is the food that is scared." - Ron Swanson