  We determine all permutation polynomials over F_{q^2} of the form X^r
A(X^{q-1}) where, for some Q which is a power of the characteristic of F_q, the
integer r is congruent to Q+1 (mod q+1) and all terms of A(X) have degrees in
{0, 1, Q, Q+1}. We then use this classification to resolve eight conjectures
and open problems from the literature, and we show that the simplest special
cases of our result imply 58 recent results from the literature. Our proof
makes a novel use of geometric techniques in a situation where they previously
did not seem applicable, namely to understand the arithmetic of high-degree
rational functions over small finite fields, despite the fact that in this
situation the Weil bounds do not provide useful information.
