Discriminant of a quadrupole with symmetric support

We will compute the discriminant of the polynomial f(x)=a+bx^k+cx^l+dx^{k+l} (where a,b,c,d are complex, k,l are natural) and discuss how it is related with dessins d'enfants on a sphere, in which one vertex is of degree 3, one vertex is of degree 1, and the rest are of degree 2.