We investigate the phase-space structure displayed by a system of four waves interacting by means of nonlinear coupling between two wave triplets, which results in a dissipative high-dimensional vector field presenting an invariant manifold, wherein the dynamics is essentially conservative. The focus is on the coexistence of a large number of periodic attractors in the phase space, with an interwoven structure of the basins of attraction, where low-period attractors have predominance. The time behavior of nearly conserved quantities and the properties of the Lyapunov spectra are used to imply the existence of a lower-dimensional invariant manifold where the dynamics is nearly conservative. A three-dimensional map is used to illustrate these findings.