Willie Nelson and John Cougar Mellencamp have announced they will hold their fourth Farm Aid concert in Indianapolis' Hoosier Dome on April 7. "I had hoped we wouldn't have to do another concert," Nelson said. "But we're losing family farms every day and many who are still on their land are just barely hanging on." . . . John Taylor, the bass player in Duran Duran, has been barred from driving for a year after he admitted driving while drunk. Taylor also was fined $400 for the offense during his court appearance in London. . . .Tennis star Andre Agassi is well aware that many of his fans are a bit younger than those usually found at major tennis tournaments. After an exhibition match with John McEnroe in Dallas last week, Agassi said: "I'm surprised to see so many of my fans here. It's a school night."