In the part with the 4 lever rooms, no combination of them all seemed to open the 4th room. They all either poisoned us, or , the one that I think is bugged, it opened itself. After it was already opened.
First room opened 2nd.
SEcond had 1 that opened the path ahead, and 1 that opened itself. Third room had one that did nothing, and we saw nothing in the first or second from it.
Always possible that we were too stupid to do it, but it felt like we had done every combination there!