There were originally supposed to be two paintings. A diptych of diptychs, if you want to be fancy about it. One for Mattew and Mark, one for Luke and John.
After I did this one, I looked up the evangelists’ associated symbols. Turned out I’d got it wrong. All four of them are shown with a book, and all four also have an associated animal.
So I left it at one picture. On the left is your generic evangelist—who happens to be Matthew—and on the right is Mark with associated, er, feline.