In fact, 4.27 is the maximum MI score for this corpus size (42,825) and the frequency of the seed term af (2,206), irrespective of the frequency of the target word, reached when all occurrences of the target word are concentrated in the window (see the appendix for details).
any hapax legomenon appearing in the window will automatically be assigned the maximum value of MI, along with target words with the most extreme W/C distributions (Window-Complement: 2-0, 3-0, 4-0, etc.).