Passing the Torch: Part 2 - S1-E19
Plot hole: In the first episode of series 4, we are told that the Power Chamber was directly below the Command Centre. In Hogday Afternoon, part 2, at the end of series 3, the Command Centre was completely destroyed - so how could the remains of the Command Centre be next door to the Power Chamber?
Answer: Scheduling problems with Hilary Shepard Turner meant that she was unable to reprise her role as Divatox from "Turbo: A Power Rangers Movie". Carol Hoyt was brought in as a replacement until the major cast change mid-season, when Hilary returned. Carol still played Dimitria, the Rangers' mentor.