Show the types of ships that have both ships with tonnage larger than 6000 and ships with tonnage smaller than 4000.
SELECT TYPE FROM ship WHERE Tonnage  >  6000 INTERSECT SELECT TYPE FROM ship WHERE Tonnage  <  4000
SELECT TYPE FROM ship WHERE Tonnage  >  6000 INTERSECT SELECT TYPE FROM ship WHERE Tonnage  <  4000
[{'Type': 'Battle ship'}]