The alleged attack took place at the Royal Agricultural University in Cirencester on 23 May.
The judge at Gloucester Crown Court told jurors the trial could not proceed at this stage.
The four accused - three aged 22 and one aged 20 - deny charges of rape and sexual assault.
A 20-year-old woman was allegedly raped and assaulted at the university end of term party.
The trial has been adjourned until Thursday.
The jury in the trial of four men accused of raping and assaulting a woman at a university summer ball has been discharged.