When I bought a plot of land to build my house on, at first I didn't buy a large enough piece, so I bought all the squares immediately surrounding it.
Once I had my house in place, I realised I didn't need that much land and went to sell some of the extra pieces.
When I went to highlight the land to sell it off, it would only let me highlight the specific chunks that I had bought in a group. It wouldn't let me highlight just the specific 4x4 squares that I wanted to sell.
I finally just sold off more than I wanted to and re-purchaced what I needed, but I think we should be able to highlight any geometry of land we want to sell.