1. Saxena, David, Jain and Kumar were District Collectors at places P, Q, R and S respectively in 1970.
2. In 1972, they were transferred. Saxena and Jain interchanged places. Kumar and David also interchanged places.
3. One year later in 1973, they were again transferred such that David and Jain interchanged places and Saxena and Kumar were also interchanged.
What should be the next round of transfer so that all the four persons could have been posted at all the four places?