Just Q missing.
Only Q, J, U, V, X, Y, Z have less than 5 airports. Other letters generally have many more airports.
Australia 19 letters.
NZ 16 letters.
USA 16 letters.
I've listed a few for NZ, Australia, USA, and some interesting other ones.
AKL, AYQ, ABQ, ADE + more
BHE, BEU, BDL, BOG +...