The trick is to make those 4 hours as productive as possible, so the rest of your day can be spent on more menial tasks. Finding the right balance comes with time and experience, so I wouldn't stress about it too much.

Sometimes people claim they can be productive for 8 or more hours per day, for weeks on end. But likely what's happening is they are tricking themselves into feeling (and looking) productive, while their actual productivity finds an equilibrium far below what it could be if they paced themselves.