The boat was travelling to Guernsey from the UK when the incident occurred on Friday afternoon (24 May).
St John Ambulance crew members met the boaters at the harbour at around 16:00 after the vessel had been steered into the marina.
Two ambulances were needed to deal with the four patients, who were initially treated at the harbour before being taken to the Princess Elizabeth Hospital.
St John said it was one of 20 calls it received last Friday.
