WASHINGTON — Setting the stage for a potentially historic ruling, the Supreme Court says it will decide whether same-sex couples nationwide have a right to marry under the Constitution. The justices said Friday they will review an appellate ruling that upheld bans on same-sex unions in four states. The case will be argued in April and a decision is expected by late June. Kentucky, Michigan, Ohio and Tennessee are among the 14 states where gay and lesbian couples are not allowed to marry. The number of …