By Michael Holden and Kylie MacLellan LONDON (Reuters) -Four of British Prime Minister Boris Johnson’s closest aides resigned on Thursday and his finance minister criticised him over a barb he made about the main opposition leader, in a tumultuous day for his government. Johnson’s premiership is facing a growing crisis in the wake of anger over a series of alcohol-fuelled parties held at his Downing Street office and residence during coronavirus lockdowns, a scandal which followed a series of other missteps. On Thursday, his Chief of Staff Dan Rosenfield, his Principal Private Secretary Martin…