With nearly two decades of experience as a trainer, I can confidently recommend that for most people looking to put on muscle mass and gain strength, the best 4-day split is an Upper Lower Split, with A & B weekly sessions, and a mix of both strength-focused lifts and hypertrophy-focused exercises (in that order).

A 5 day split is considered to be an excellent workout split for building muscle. It allows for maximum training volume and emphasis on each major muscle group, while also providing sufficient rest days each week. This is particularly beneficial for advanced trainees, as one day of rest per week is generally not enough for most individuals.

There are a few good workout splits for building muscle, including: Full-Body Workouts: These workout routines stimulate most muscles during most workouts. Upper/Lower Splits: These routines split your workouts into lower-body and upper-body exercises. Push/Pull/Legs Splits: This is when you divide your workouts into pushing, pulling, and legs.