  Branches of moduli space of F-theory in four dimensions are investigated. The
transition between two branches is described as a 3-brane-instanton transition
on a 7-brane. A dual heterotic picture of the transition is presented and the
F-theory - heterotic theory map is given. The F-theory data - complex structure
of the Calabi-Yau fourfold and the instanton bundle on the 7-brane is mapped to
the heterotic bundle on the elliptic Calabi-Yau threefold CY_3. The full moduli
space has a web structure which is also found in the moduli space of
semi-stable bundles on $CY_3$. Matter content of the four-dimensional theory is
discussed in both F-theory and heterotic theory descriptions.
