LAGROBO is a model for fitting analytical potential energy surfaces (PES) in which the potential function is expressed as a many-process expansion of rotating bond order (ROBO) potentials. The model is developed in bond order (BO) coordinates and shows several advantages. In particular, it can be used to make PESs for chemical reactions for which only limited ab initio information is available. Furthermore, it ensures a smooth potential surface and can describe all the different arrangement channels in a many-atom system. So far the LAGROBO model has been applied to three-atom systems. We report the formulation of the model for four-atom reactions. As an example, an analytical PES for the OH + H-2 system has been assembled.