We study F-theory duals of six dimensional heterotic vacua in extreme regions
of moduli space where the heterotic string is very strongly coupled. We
demonstrate how to use orientifold limits of these F-theory duals to regain a
perturbative string description. As an example, we reproduce the spectrum of a
$T^4/\ZZ_{4}$ orientifold as an F-theory vacuum with a singular $K3$ fibration.
We relate this vacuum to previously studied heterotic $E_8\times E_8$
compactifications on $K3$.
