  We investigate a 4D analog of 2D WZW theory. The theory turns out to have
surprising finiteness properties and an infinite-dimensional current algebra
symmetry. Some correlation functions are determined by this symmetry. One way
to define the theory systematically proceeds by the quantization of moduli
spaces of holomorphic vector bundles over algebraic surfaces. We outline how
one can define vertex operators in the theory. Finally, we define
four-dimensional ``conformal blocks'' and present an analog of the Verlinde
formula.
