Database: toxicology
Question: Tally the toxicology element of the 4th atom of each molecule that was carcinogenic.
SQL: SELECT DISTINCT T1.element FROM atom AS T1 INNER JOIN molecule AS T2 ON T1.molecule_id = T2.molecule_id WHERE T2.label = '+' AND SUBSTR(T1.atom_id, -1) = '4' AND LENGTH(T1.atom_id) = 7