Humberside Police said they were in their tent on grassland in Hull when they were set upon in the early hours of Tuesday, October 9.
One of the victims suffered a fractured spine in the attacks, said to have happened between Fountain Road and Stepney Lane.
A police spokesperson said: "It was reported that a group of four men had assaulted the pair in the tentat around 2.30am and then again at 5am.
"One of the victims sustained a fractured spine, a fractured arm, bruising and facial injuries. The other man sustained bruised ribs and facial injuries.
"Three of the suspects have been described as having dark hair, the fourth had blonde hair. They spoke with Russian or possibly Lithuanian accents."
Witnesses should call police on 101.