  We investigate the perturbative and non-perturbative correspondence of a
class of four dimensional dual string constructions with N=4 and N=2
supersymmetry, obtained as Z_2 or Z_2 x Z_2 orbifolds of the type II, heterotic
and type I string. In particular, we discuss the heterotic and type I dual of
all the symmetric Z_2 x Z_2 orbifolds of the type II string, classified in
hep-th/9901123. .
