CREATE TABLE table_50757 ("Class" CLOB, "Wheel arrangement" CLOB, "Fleet number ( s ) " CLOB, "Manufacturer" CLOB, "Year made" CLOB, "Quantity made" CLOB, "Quantity preserved" CLOB)

What is the quantity preserved when 4 were made of class F-21?
SELECT "Quantity preserved" FROM table_50757 WHERE "Quantity made" = '4' AND "Class" = 'f-21'
SELECT "Quantity preserved" FROM "table_50757" WHERE "Class" = 'f-21' AND "Quantity made" = '4'