On the collision of two projectiles on two targets in the BFKL approach

High-energy collisions of two nucleons on two nucleons are studied in the BFKL approach in the leading approximation in ${\alpha }_s{N}_c$. Diagrams with redistribution of colour are considered. It is found that intermediate BKP states consisting of 4 reggeized gluons give a contribution which may be leading in deuteron-deuteron scattering and thus experimentally observable.