The game is supposed to be both. Sims 1, 2, and 3 were all a blend of life sim and sandbox. Sandbox only means you can play how you want without quests or linear play. The life sim part should just be about simulating life.
Sims 4 fails hard at simulating life. Easy examples of this are the walk-bys who should be at work or school show up. Someone posted a picture on Reddit the other day of a toddler assigned as a doctor. Adults call children in the game and invite them to go out. The cat calls to tell you they're sorry for your loss. Infants call to congratulate your ability to make a friend. None of those things would have ever happened in previous games. Sims 4 was designed to be a virtual chatroom with sims and houses. The game mechanics were added later and in under a year. That's why this game fails so hard to do the one job it has.