Rule 4 is a scoring rule that allows only one catcher to be on the field at one time and sets the procedure for deciding the number of rounds.
Summary
Each team will get a set number of rounds, only their catcher can be in the game (so they can control when it ends).
Official Rule
Each team will have an equal and previously decided number of rounds. Only their catcher, and not the other team's, can have thier catcher on the field.