Four men were arrested in Dorchester on various drug and firearm charges Thursday after complaints from residents about drug activity prompted police to investigate, Boston police said. Officers conducting surveillance saw a woman buy what they believed to be drugs from Andrew Malabre, 22, of Mattapan, and arrested him in front of a house on Irma Street, police said. A man on the porch then allegedly stood up and clutched his groin, prompting officers to search him, police said. The man, Roosevelt R. Thornton, 22, of Roxbury, was found with a loaded gun, police said. Justin Haynes, 27, of Mattapan, and Richard Griffiths, 20, of Brockton, were in possession of drugs, police said. A loaded handgun was also discovered hidden in a speaker on the porch, police said. Authorities did not say when the men would be arraigned.