You have two options if you are looking to reduce your training volume. If you don’t have five days per week to train, choose three or four workouts per week and rotate which days you choose evenly. That way you get a dose of everything. Functional Bodybuilding will hit different movement patterns throughout the week, so choosing a mix of days as well as keeping an eye on what movement patterns you’re training will ensure sufficient variety. Example: if you did lots of horizontal and vertical pressing last session, choose a session that focuses on lower body such as squatting, hinging, etc.
Another option is to pick the days that target the skills and/or muscle splits you are looking to focus on and keep those days the same each week. Since muscle splits and skill days remain the same week to week within a cycle, this will ensure maximal results if you have a specific goal within your training.
As always, Pillars might be a great option for those who want to train four days a week and do not want the stress of reducing Pump or Perform down.