A simple solution is to allow the teams of 3 after finishing to select a 4th player at random from the teams of 4. The downside is that it means everyone in the 4 balls has to finish each hole if they can score and you have to trust them to do their best, rather than sat 3 putting on purpose because their teams scores are secured (if playing best 2 or 3 stableford scores from 4).
The upside is the added drama in the clubhouse of selecting the additional players to find out who's won.