The heightened security for the Republican convention resulted in four Pakistani students from St. John's University being detained in Times Square last night. The four, who were in a burgundy van, were taken into custody shortly after 5 p.m. outside the Marriott Marquis hotel on Broadway and West 45th Street after cops thought they were acting suspiciously. They were questioned for at least seven hours at the Midtown North precinct on West 54th Street while anxious relatives waited outside. Police sources said two of the students were on an FBI watch list.