LONDON  Spurred by false racist rumors, a fourth consecutive night of violence has plunged Northern Ireland into crisis, as anti-immigrant rioters once again attacked police, throwing bricks and masonry. The unrest, triggered by the arrest of two local teenage boys in connection with an alleged sexual assault earlier this week, continued on Thursday night. A crowd threw bricks and masonry at officers in Portadown, County Armagh amid a heavy police presence. Some 400 protesters reportedly