The four passengers joined another four already in the hospital in the Rhine River town of Boppard suffering from an outbreak of the highly contagious norovirus, putting the total at eight.
On Tuesday the captain of the Lady Anne, which was carrying 110 passengers and crew on a cruise up the picturesque Rhine, called for emergency help after dozens of people on board began experiencing severe nausea, vomiting and diarrhea. Authorities placed the ship under quarantine in Boppard and no one was allowed on or off the boat without permission.
Around 43 people on the ship appear to have been infected with the virus, according to the River Cruise Line, the Leicester, UK-based company which operates the ship. A German health official said all the vacationers were at least 70-years-old, the BBC reported.
The company said in a statement the quarantine has been lifted and that those passengers who were fit to travel would be returning home on Wednesday, accompanied by a doctor. Passengers will get a full refund.
The Lady Anne will be professionally cleaned to ensure the safety of future passengers, The River Cruise Line said.
"We would like to again express our gratitude for the excellent work of the German paramedics and offer our apologies to all those directly and indirectly affected," the company said.
The source of the infection is not yet known. The norovirus is the most common cause of gastroenteritis around the world and can be transmitted easily through contact with an infected person; by consuming contaminated food or water or by coming into contact with contaminated surfaces or objects.
The virus can cause sudden nausea, projectile vomiting and watery diarrhea.
Most people who have been infected make a full recovery within one to two days. But the very young or elderly are at greater risk of becoming dehydrated and may require hospital treatment. Experts recommend that those who have been infected be isolated for up to 48 hours after symptoms have disappeared.