I was reseated in F. In both cases my seat assignment mysteriously changed online to another F seat. When I called in to ask about it they said it had been done for "security reasons." I remembered to look at the person in my old seat when I boarded and sure enough in both cases the guy looked the part.

I don't know if 1 and 4 are exempt in some way. I do tend to try and stay away from row 4 on routes where a plane with just 3 F rows might be swapped in.