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

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'