  The aim of this article is to introduce invariants of oriented, smooth,
closed four-manifolds, built using the Floer homology theories defined in two
earlier papers (math.SG/0101206 and math.SG/0105202).
  This four-dimensional theory also endows the corresponding three-dimensional
theories with additional structure: an absolute grading of certain of its Floer
homology groups. The cornerstone of these constructions is the study of
holomorphic disks in the symmetric products of Riemann surfaces.
