Episode #4.3 - S4-E3
Plot hole: The killer poisons the victim's belonging when they met early in the morning - that means that the victim stayed the whole day with the poison in the pages of his journal he always brings with him. Yet the murderer's plan required the poisoning to happen at a very specific and precise moment: the end of the day after they'd all had eaten together the stew with the non-lethal dose. It is something that they couldn't have planned at all the way it is described, not when they did the poisoning so far in advance.