It’s assumed 4 are being used for family worship, so the assumption is they are doing that, but only getting in 6 hours in the field ministry.
Many years ago my dad was booted off the body for getting "only" 5 hours per month in the "ministry". They deducted the 4 hours for family study, plus the 16 hours per month in weekly bible studies with persons in the "field". I only wish I could have completed helping him write the Borg out of his will before he suddenly died.