It's helped me figure out the misunderstanding, thanks!
The problem was, when I read "Land Area/500 (4.00) tons of food per turn", I thought it meant "Land Area/500 × (4.00) tons of food per turn". I understand now it means "Land Area/500 (i.e. 4.00 in this city) tons of food per turn", right? This is where my extra "× 4" was coming from.
All right, so I'll dismantle these farms and rebuild the mines I had before…