Translate the following question to SQL: Show the types of ships that have both ships with tonnage larger than 6000 and ships with tonnage smaller than 4000.

Schema: CREATE TABLE mission (
    Mission_ID INT   PRIMARY KEY,
    Ship_ID INT,
    Code TEXT,
    Launched_Year INT,
    Location TEXT,
    Speed_knots INT,
    Fate TEXT,
    UNIQUE (Mission_ID),
    FOREIGN KEY (Ship_ID) REFERENCES ship(Ship_ID)
);

CREATE TABLE ship (
    Ship_ID INT   PRIMARY KEY,
    Name TEXT,
    Type TEXT,
    Nationality TEXT,
    Tonnage INT,
    UNIQUE (Ship_ID)
);
user
SELECT TYPE FROM ship WHERE Tonnage  >  6000 INTERSECT SELECT TYPE FROM ship WHERE Tonnage  <  4000
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