It is well-known that topological σ-models in two dimensions constitute a path-integral approach to the study of holomorphic maps from a Riemann surface Σ to an almost complex manifold K, the most interesting case being that where K is a Kähler manifold. We show that, in the same way, topological σ-models in four dimensions introduce a path integral approach to the study of triholomorphic maps q : M → N between a four dimensional Riemannian manifold M and an almost quaternionic manifold N . The most interesting cases are those where M, N are hyperKähler or quaternionic Kähler. BRST-cohomology translates into intersection theory in the moduli-space of this new class of instantonic maps, that are named by us hyperinstantons. The definition of triholomorphicity that we propose is expressed by the equation q * − J u • q * • j u = 0, where {j u , u = 1, 2, 3} is an almost quaternionic structure on M and {J u , u = 1, 2, 3} is an almost quaternionic structure on N . This is a generalization of the Cauchy-Fueter equations. For M, N hyperKähler, this generalization naturally arises by obtaining the topological σ-model as a twisted version of the N=2 globally supersymmetric σ-model. We discuss various examples of hyperinstantons, in particular on the torus and the K3 surface. We also analyse the coupling of the topological σ-model to topological gravity. The classification of triholomorphic maps and the analysis of their moduli-space is a new and fully open mathematical problem that we believe deserves the attention of both mathematicians and physicists.