Get another possession if you fail is way different than going for it in OT.

If KC gets the ball first and they have 4th and 4 at their 31 yard line, KC more than likely punts and plays defense to stop the Niners from trying to get a FG.

Once the Niners scored a FG on their first possession, KC had 4 downs to get a first down or score so it is irrelevant to what the down and distance on 4th down is or where on the field KC is at because they are going for it on 4th down in order to extend the game.

I believe KC did convert a 4th down at midfield on that final OT drive to win.