Hard to say. There are strange reasons for certain mathematicians being passed on the Fields Medal. According to the Langlands memorial talk, Harish Chandra was passed over (1958) because a member of the committee felt he was "too much of a Bourbakite". Maybe there were strange political reasons for Arnold being passed over as well.

Personally, it's a mystery why so many brilliant mathematicans like Gromov(!), Manin, Beilinson, etc. have been passed over. Mathematics is such a broad subject, filled with so many really smart people, I would think it would not be hard to find four mathematicians who satisfy the requirements for the medal.