Below is an instruction that describes a task. Write a response that appropriately completes the request.

### Instruction:
what is the class when the quantity is 4?

### Inputs:
table_name: table, columns: ['Class', 'Railway number(s)', 'Quantity', 'Year(s) of manufacture', 'Axle arrangement ( UIC )']

### Response:
SELECT Class FROM table WHERE Quantity = 4