Traditional statistical approaches, entirely based on transition state theory (TST), do not allow the description of rotational state distributions in the products of indirect reactions governed by short-range forces. Owing to the interpretative power of TST, this limitation has long been acting as a brake upon a deep understanding of determining attributes of indirect reaction dynamics. Recently, however, we developed a statisticodynamical approach (SDA) of final state distributions for triatomic unimolecular reactions [P. Larregaray, L. Bonnet, and J. C. Rayez, J. Chem. Phys. 114, 3349 (2001); Phys. Chem. Chem. Phys. 4, 1577 (2002); 4, 1781 (2002)]. The approach combines TST for the description of state distributions at the transition state (TS) and the linear transformation model for the description of their alteration on the way from the TS to the products. The whole description is mostly analytical, thus keeping the rationalizing spirit of TST. The goal of the present series is to extend SDA to the case of four-atom planar unimolecular reactions, assuming that internal vibrations of the nascent products are conserved from the TS on. This first part is concerned with formal developments while the remaining parts deal with their validation and application, in particular to the fragmentation of isocyanic acid. (C) 2004 American Institute of Physics.