We propose a way of computing 4-manifold invariants, old and new, as chiral
correlation functions in half-twisted 2d N=(0,2) theories that
arise from compactification of fivebranes. Such formulation gives a new
interpretation of some known statements about Seiberg-Witten invariants, such
as the basic class condition, and gives a prediction for structural properties
of the multi-monopole invariants and their non-abelian generalizations.Comment: 67 pages, 11 figure