On most trains, including the Shinkansen bullet train, you can rotate the the seats, or move the back from one side to the other.
Not total freedom… you can’t sit in the aisle or on top of someone else… but you can turn them from facing forward to back.
All the annoying things about train seats disappear. If your group of four want to face each other and chat, just move the seat back from one side to the other. If you want to face forwards instead of backwards (and there’s no one sitting there) you can just do it.
Reached your destination… train staff walk through the train and turn all the seats to face the direction of travel. Everyone’s happy.