As many as 30,000 doormen, superintendents and porters in Manhattan, Brooklyn, Queens and Staten Island could be leaving their posts by the end of the month, according to Crain’s, as the workers’ union vote on whether they’ll strike over contract disputes. The latter three boroughs voted this week to strike if an amenable contract agreement hasn’t been reached by April 21. The Manhattan workers are expected to vote tonight. The possible strike comes after the union rejected the Realty Advisory Board’s proposed contract in March. Should the strike go through, approximately 3,200 buildings could see their staffers walk.