United Press International yesterday announced a new team of top editors, the fourth group to run the struggling news service since its purchase last year by Mexican businessman Mario Vazquez Rana.

Al Rossiter Jr., a UPI science editor, was appointed executive editor. Three managing editors were also named. Bill Ferguson, who has been a top UPI manager for 20 years, was appointed managing editor of national news. Leon Daniel, a longtime UPI foreign correspondent, will be managing editor of foreign news. Michael Freedman, director of the UPI Radio network, will succeed Ferguson as managing editor of broadcast news.

In appointing Rossiter, Ferguson and Daniel, Vazquez Rana has tapped newsmen who have been with UPI since the 1950s, in contrast to his last top editorial team, which came from The Washington Post and Newsweek.

That team -- Ben Cason, the wire service's editor; Barry Sussman, managing editor for national news, and Kim Willenson, managing editor for international news -- quit Nov. 3 to protest financial cutbacks. They said they could "no longer assure the quality and integrity of the UPI report," and said that UPI planned to lay off 300 of its 1,400 employes, a charge that UPI spokesman Christopher Smith has flatly denied.

Vazquez Rana has had a difficult time stabilizing the management of UPI since purchasing the wire service, which was then in bankruptcy proceedings, in June 1986. Industry sources have speculated that Vazquez Rana will close down or sell the news service, but UPI has denied such rumors.

Rossiter, 51, who will head the operation, joined UPI in 1959. He managed UPI's Cape Canaveral bureau and was named science editor in 1973. Ferguson, 61, joined UPI in 1950. He was named national editor of the broadcast news division in 1967 and managing editor of broadcast news in 1986. Daniel, 56, joined UPI in 1956 and has covered the Vietnam War, the Grenada invasion and the Persian Gulf War for UPI. Freedman joined UPI in 1986 and had been news director of WCXI AM-FM in Detroit for five years.