How this rotation runs
Every 20 minutes the guard on break takes the deep end, the deep-end guard moves to the shallow end, the shallow-end guard moves to the slide, and the slide guard goes on break. Each guard watches for exactly an hour, then gets 20 minutes off. That is right at the 60-minute limit in the Model Aquatic Health Code, so don't stretch the rotation past 20 minutes with this many stands.
A fifth guard turns the break into 40 minutes. A fourth stand with the same four guards leaves nobody a break, since every stand needs a guard at all times. With 5 guards and 4 stands at 20 minutes, guards would be up 80 minutes, so rotate every 15 minutes or add a sixth guard and spread the breaks.
Variations
Other setups
Rotation limits from the CDC Model Aquatic Health Code (6.3.3.1.2). Your facility's rules and certifying agency come first.