What are the types of the ships that have both shiips with tonnage more than 6000 and those with tonnage less than 4000?
return ships ;return tonnages of  #1 ;return #1 where #2 is  higher than 6000 ;return #1 where #2 is  lower than 4000 ;return #1 of  both #3 and  #4 ;return types of  #5
['select', 'project', 'comparative', 'comparative', 'intersection', 'project']