The mother of a severely malnourished 4-year-old found dead last week in Brooklyn would beat her tyke with a belt and force-feed the child sleeping pills, authorities said yesterday. Carlotta Brett-Pierce was especially furious with her daughter, Marchella, because the girl, who weighed a mere 18 pounds when she died, would get up at night and "eat from the fridge and make a mess," said prosecutor Tracey Downing at a Brooklyn Criminal Court hearing.