Has an NPB (Japan) game ever ended 15-4?
It has. 15-4 has been a NPB (Japan) final 33 times in 63,911 games. The first was Kintetsu Pearls beat Daiei Stars on April 9, 1950; the latest was Tohoku Rakuten Golden Eagles beat Chiba Lotte Marines on June 30, 2020.
That makes 15-4 the 101st most common final score out of 223 that have ever occurred in NPB (Japan). One point away, 16-4 has happened 23 times and 15-5 has happened 25 times.