Two women said witches can change appearance, and two women say they cannot. Since two women are innocent and will always tell the truth, that means that the innocent women agree on this statement, and the other two are lying about it.
Wilhelmina and Beatrix are innocent and telling the truth. Witches can appear like other women. Wilhelmina saw Heddy and Beatrix in the town hall because Rosemary (the witch) changed her appearance to look like Beatrix. Heddy is the accomplice. The keyword is ROSEMARY.