If the signal looked fine while flying and recording, but is twitchy while playing that video back on the device (tablet / phone on the controller, or the screen of a Smart Controller), then that is not normal / usual and isn't something I've seen reported much here before.
Note: I asked the questions to be sure that it wasn't simply an Ocusync transmission problem between the aircraft and the controller, which will sometimes show drop-outs that do not show up on the actual recording.
I would format your card in the aircraft (via the Format command in DJI Go 4) and try some tests again. If it is still twitchy on playback of recorded 4K video, I would try another card. If the 2nd card is fine, you may have a faulty or counterfeit 1st card. If they are both showing the same twitchy playback, then you may have a problem with your aircraft, such as a bad camera / gimbal connection (damaged, or just a faulty factory defect), in which case I would a) return it if it's a new purchase or b) send it in for repairs.