[SAump]

The fourth out may involve a missed base appeal. The B/R who misses a base allows for the possibility of a 4th out to be made on appeal by the defense. The fourth out resulting from a missed base appeal "force-out" often wipes off any runs scored during the playing action.

During a live ball, the 3rd out made on a timing play most often ends an inning. However in the OP, the 3rd out {a timing play} did not end this inning. The BR who never touched 1B allows the defense an opportunity to complete a legal inning ending double play. This advantageous fourth out wipes the run off the board.