I think this is key here. Being fully focussed for four hours is unrealistic.
This makes a lot of sense, particularly if you think you are losing focus when you are distracting yourself between shots.
You mention hydration and snacks so you maybe have this covered, but your focus/decision-making ability deteriorates rapidly without adequate hydration especially. In the case of both nutrition and water, a rule that is often applied in endurance sports is "If you feel hungry/thirsty, it's too late". I expect the best nutrition sources/pattern for golf are discussed elsewhere, but it sounds from your description like you would benefit from taking a look at what/how much you are eating prior to and during your round.