Has a World Baseball Classic game ever ended 4-0?
Exactly once. On March 5, 2009, Japan beat China 4-0, and no World Baseball Classic game before or since has ended that way. When it happened it was a scorigami, the first 4-0 in league history.
That makes 4-0 the 47th most common final score out of 80 that have ever occurred in World Baseball Classic. One point away, 5-0 has happened 5 times and 4-1 has happened 9 times.