Tears of Itloacol
What happened in this case is, that you were not actually missing the 4th but the 3rd combination. In some cases the left Symbol from the first round will still show, when the 3rd and 4th are added, so they are overlayed. Your best bet to solve this in any state is just to try all possible shapes (about 10 iirc). They can be found on the corresponding wiki page.