The workout split refers to a program or routine that divides your training sessions between each muscle group. It is mostly used by professional bodybuilders and professional powerlifters to ensure that they exercise each muscle group adequately.
The rule of thumb is that each muscle group should be exercised twice within 7 days. This concept does not follow your general Monday-Sunday days, the muscle groups just need to be exercised twice per seven days. For instance, if you exercise legs and glutes on Monday, you’ll need to exercise your legs and glutes again before Sunday the same week. Or if you exercise chest on Wednesday, then you’ll need to exercise your chest again by Wednesday next week. You will not need to exercise it again by Sunday the same week.
Once the 4-day workout split is complete, a rest day should be taken or the cycle should be restarted and repeated the cycle. When the cycle is repeated, you want to vary your exercises or use progressive overloading to make sure that the body doesn’t adapt to the workouts that are being performed.