Should I call an ambulance if I mistakenly took 4 tablets of Xifaxan at once?

Doing so does not lead to an overdose. However, symptoms from the digestive system may intensify, e.g. nausea, vomiting, diarrhea. Nevertheless, this is not a life-threatening situation and you do not need to call the emergency room. According to the manufacturer: "In clinical trials of patients with travellers' diarrhoea, doses up to 1800 mg/day were tolerated and there were no serious side effects. Even in patients/subjects with normal bacterial flora, administration of rifaximin 2400 mg/day for 7 days did not result in significant clinical signs associated with the high dose." More information about Xifaxan can be found here: https://www.gdziepolek.pl/produkty/84652/xifaxan-tabletki-powlekane/apteki