  We investigate the differential geometry of the moduli space of instantons on
S^3 x S^1. Extending previous results, we show that a sigma-model with this
target space can be expected to possess a large N=4 superconformal symmetry,
supporting speculations that this sigma-model may be dual to Type IIB
superstring theory on AdS_3 x S^3 x S^3 x S^1. The sigma-model is parametrized
by three integers -- the rank of the gauge group, the instanton number, and a
"level" (the integer coefficient of a topologically nontrivial B-field,
analogous to a WZW level). These integers are expected to correspond to two
five-brane charges and a one-brane charge. The sigma-model is weakly coupled
when the level, conjecturally corresponding to one of the five-brane changes,
becomes very large, keeping the other parameters fixed. The central charges of
the large N=4 algebra agree, at least semiclassically, with expectations from
the duality.
