Say the only score boxes you have available in the middle of a Yahtzee game are fives and fours from the upper section and it's your turn. Your initial roll gives you one five and one four, the rest being something else. For the next throw, which dices would be the best to save (not throw again) for maximizing the chances for best points for the rest of the game?
You may disregard the chance for the upper section bonus (the player has already received it or is ineligible for it) and Yahtzee bonuses. The only ways to get points at this point would be from the fives and fours themselves.
The game ends when there are no players with available score boxes. Because this question is about optimizing one's own score, it should be irrelevant whether the game is played solitaire or with other players. A Yahtzee turn consists of initial dice roll (all five dices are rolled) and two corrective rolls, where the player can freely choose which dices they want to roll and which to save. Only the initial roll is required and the player may choose to register their points to any available score box at any point after the initial roll, meaning after the initial roll, after the first corrective roll or after the second corrective roll. The turn ends when a score box is used and becomes unavailable. Score box of fives gives n * 5 points where n equals the number of fives in the player's dice row and score box of fours gives n * 4 points where n equals the number of fours in the player's dice row.