In addition to 4K, most default camera apps offer some kind of slow-motion mode, which can be useful for certain scenes. On Android, access it from the main app menu; on iOS, pick it from the carousel of modes along the bottom. Pay attention to the frame rate of the slow-motion mode: The slower you want to go, the higher the frames-per-second (fps) will be. While this might be desirable for super-slow-mo, higher fps also goes with lower image resolution. If you can't find a reference to FPS in the app itself, look it up online, and decide whether the compromise between frame rate and resolution is worth it. You will also need a lot of light to shoot in slow-motion, so avoid it in dark situations.