Bishop Bernard Fellay of the Saint Pius X Society, a Swiss-based Catholic fraternity, ordained the four men at a seminary in Zaitzkofen, near the Bavarian city of Regensburg, on Saturday.
Williamson’s appeal is to be heard in Regensburg district court on Monday.
Priests ordained by the rebellious arch-traditionalist brotherhood are not recognized by the Catholic Church.
The controversial brotherhood was stripped of ordination rights by the Vatican after they refused to recognise a raft of modernising changes to the church in the late 1960s.
Among the reforms rejected by the organization was a declaration called Nostra Aetate, which ended a Catholic Church doctrine by which the Jews were held responsible for killing Jesus.
In 2008 Williamson, who is British, gave an interview to a Swedish TV station during which he questioned the historical details of the Holocaust. The interview, which was aired in Sweden in January 2009, took place at the Pius Brotherhood seminary at Zaitzkofen.
“I believe that the historical evidence, the historical evidence, is hugely against six million Jews having been deliberately gassed in gas chambers as a deliberate policy of Adolf Hitler,” Williamson said in the interview.
“I believe there were no gas chambers … I think that 200,000 to 300,000 Jews perished in Nazi concentration camps, but none of them by a gas chamber.”
In April 2010 he was found guilty of Holocaust denial, a hate crime in Germany, and fined €10,000.
Pope Benedict XVI severely criticised for reversing the excommunication of Williamson and three other Saint Pius X Society bishops in 2009 – announcing his decision just days after the interview was broadcast.
The pope stressed that he had been unaware of Williamson’s remarks when making his decision, insisting it was “intolerable” to dispute the facts of the Holocaust.