Collision induced dissociation and four center exchange reaction in H-2(v(1)=high)+H-2(v(2)=low) are studied by means of time-dependent wave packet calculations and within a three-degree-of-freedom reduced dimensionality model. The role of both-vibrationally excited and vibrationally cold-collision partners is examined varying v(1) between 10 and 14 and v(2) between 0 and 2, respectively. From the analysis of the results, a clear picture of the main mechanisms of dissociation and reaction has been obtained, and the regions of the potential energy surface most sensitive to the dynamics have been identified. In this way, reaction bottlenecks are found to significantly depend on the initial v(1) state, owing to the anharmonicity of these v(1) states near dissociation and the different regions of the potential explored by the associated wave packets. The topography of such bottlenecks provide a basis for the existence of tunneling in (v(1)=10,12, v(2)=0-2) reactions. Regarding the dissociation process, we find that there are two main mechanisms for dissociation; one where the unbroken diatom recoils with respect to dissociated fragments, and the second, where the diatom passes through the dissociated fragments. These mechanisms are responsible of a double peak observed in some dissociation probabilities. For (v(1)=14, v(2)=0-2) reagents, new processes appear with non-negligible probabilities: (i) inelastic collision by insertion of the cold diatom into the vibrationally excited one and (ii) dissociation of the initially cold diatom. These features, together with the observation of structures in all channel probabilities, suggest that four-atom complexes are formed during collision.