One week contains 7×24=168hrs.
If each employee is to have an equal share of covering these hours=168/4=42hrs/employee per week. This is equivalent to 42/7=6 working hours per day for each employee (that is, excluding break times). An employee would be in work for perhaps 49 hours a week when breaks are included. This arrangement also means that the employees all work at weekends.
Let's assume that the minimum time off between shifts is T hours and that b hrs is the break time within each shift.
The time in work for each employee=6+b hrs. 24-(6+b) is the time between shifts for the same employee so 24-(6+b)≥T, that is, the non-working hours must be at least as large as the minimum time off between shifts. Another consideration may be the travel time to and from work for each employee. This will probably be different for each employee, so some nominal commute time (C) allowance needs to be applied. Then we have 24-(6+b+C)≥T. Let b=1hr and C=3hrs then 24-10≥T, 14≥T. If T=12hrs then each employee would be able to to work 6 hours a day with a total of one hour in breaks. Each employee would have an estimated maximum of 3 hours' total commute time per day to and from the workplace. And each employee would have about 14 hours between daily shifts.
If four employees cover 24 hours, then each covers 6 hours. Two half-hour breaks or a single one-hour break can be included so the employee is actually at work for 7 hours. If employees arrive every 6 hours then there will be one hour of overlap which could be used to relay information from one shift to the next. The breaks would create a lack of cover for the period of the break.
This is a very simple arrangement, and a lot of other factors have not been taken into account, such as the statutory working-hours limit, fluctuations of work demand through the day, etc.
