Nebraska Sen. Bob Kerrey, chairman of the Democratic Senate Campaign Committee, spent the day downplaying reports that he said four more Senate Democrats will retire next year, leaving a threatening seven seats open toGOP challengesin 1996. In Thursday's editions of the Capitol Hill newspaper Roll Call, Kerrey was quoted as naming Sens. Sam Nunn of Georgia, Howell Heflin of Alabama, Claiborne Pell of Rhode Island and David Pryor of Arkansas as potential retirees. Today, Kerrey demurred: "There are additional members who have not decided whether or not they're going to run for re-election." Pell, 76, said he hadn't yet decided. Nunn, 56, has $1.3 million in a re-election war chest but has dropped hints he may leave. Heflin, 73, has medical problems, and Pryor, 60, is keeping mum for the moment.