  For the proposed duality relating a family of N=4 superconformal coset models
to a certain supersymmetric higher spin theory on AdS_3, the asymptotic
symmetry algebra of the bulk description is determined. It is shown that,
depending on the choice of the boundary charges, one may obtain either the
linear or the non-linear superconformal algebra on the boundary. We compare the
non-linear version of the asymptotic symmetry algebra with the non-linear coset
algebra and find non-trivial agreement in the 't Hooft limit, thus giving
strong support for the proposed duality. As a by-product of our analysis we
also show that the W_infinity symmetry of the coset theory is broken under the
exactly marginal perturbation that preserves the N=4 superconformal algebra.
