PRISTINA - Kosovo marked the fourth anniversary of its declaration of independence Friday under a cloud as it struggles to assert authority over defiant ethnic Serbs and tackle rampant corruption. Just two days before the anniversary 99.74 percent of Serbs in northern Kosovo, which borders Serbia proper, rejected the ethnic Albanian government in a referendum meant to highlight their anger at concessions Belgrade made in talks with Pristina. Although the vote had no significant impact on the political scene.