what is the class when the quantity is 4?
CREATE TABLE table_64251 (
    "Class" text,
    "Railway number(s)" text,
    "Quantity" real,
    "Year(s) of manufacture" text,
    "Axle arrangement ( UIC )" text
)
SELECT "Class" FROM table_64251 WHERE "Quantity" = '4'
Below are sql tables schemas paired with instruction that describes a task. Using valid SQLite, write a response that appropriately completes the request for the provided tables. ### Instruction: what is the class when the quantity is 4? ### Input: CREATE TABLE table_64251 (
    "Class" text,
    "Railway number(s)" text,
    "Quantity" real,
    "Year(s) of manufacture" text,
    "Axle arrangement ( UIC )" text
) ### Response: SELECT "Class" FROM table_64251 WHERE "Quantity" = '4'