The 4Rs indicate how any on-the-ball skill is integrated into the flow of a point.  Each of the four phases of a skill are needed to execute an efficient and effective skill. 

1. The preparatory phase means that the player has positioned herself in the court in relation to the shot she has played and is READING to see how the opponent responds to her shot.  Any movement from or staying in the BASE position is known as DECISION MAKING.
2. The wind-up phase means that the player RESPONDS to what she reads as the opponent's intent, moving to COVER the target area of the shot and shifting her weight and preparing the racquet arm ready for the stroke she has selected.
3. The force producing phase of the skill is where he player executes the stored up energy (weight, elastic energy) into a shot to execute a stroke onto the ball.  A critical aspect to make the shot successful is minor movements to ADJUST the body position in relation the ball (hitting zone), or as the stroke is executed to ADJUST to a ball being struck in a less than ideal hitting zone.  In this way the player REACTS to the play of the ball in the court.
4. Follow-through phase of the skill is where the player RECOVERS from the previous skill into a ready position (knees bent, feet shoulder width apart, weight forward, racquet held in two hands, grip relaxed), regaining stability as she moves into the preparatory phase for the next skill.

A more detailed description of the 4Rs model can be found in the following article