If someone can only train 4 days a week I would normally add a quad exercise on shoulder day and a hams exercise on back day.
Everything gets hit directly or indirectly twice a week with proper exercise selection
Triceps are actually hit 3x (with chest and delts work as a synergist in pressing movements and on nack/triceps day)
Biceps are hit twice (chest/biceps and as a synergist on pulling movements)
Shoulders are hit twice (delts day and as a synergist on chest exercises)
Back is work twice or even three times (back day, shoulder day… rear delts exercise work the rear delts and rhomboids and on posterior chain day since you should do deadlifts)
The only muscle receiveing less stimulation is the chest, although the upper chest does work on overhead pressing exercises (and I also often use high incline press for delts) and is a synergist on properly done pull-ups… you can add pullover on back day which also works the chest.