user: Question: What is the quantity preserved when 4 were made of class F-21?
Columns: ['Class', 'Wheel arrangement', 'Fleet number(s)', 'Manufacturer', 'Year made', 'Quantity made', 'Quantity preserved']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['2-8-0 — oOOOO — Consolidation', '2-8-0 — oOOOO — Consolidation', '2-8-0 — oOOOO — Consolidation', '2-8-0 — oOOOO — Consolidation', '2-8-0 — oOOOO — Consolidation', '2-8-0 — oOOOO — Consolidation', '2-8-0 — oOOOO — Consolidation']
SQL:

assistant: SELECT "Quantity preserved" FROM "Table" WHERE "Quantity made" = '4' AND "Class" = 'F-21';