List the capability of research postgraduate students with an intellegence level of 4 and above.
research postgraduate students refers to type = 'RPG'; intelligence level of 4 and above refers to intelligence > = 4;
SELECT T1.capability FROM RA AS T1 INNER JOIN student AS T2 ON T1.student_id = T2.student_id WHERE T2.type = 'RPG' AND T2.intelligence >= 4