The company purchased a machine for $40,000. The machine had an estimated residual value of $4,000 and an estimated useful life of nine years. After three full years of experience with the machine, it was determined that its total useful life would be only seven years instead of nine. In addition, a revised estimate of $8,000 was made for the residual value, instead of the original $4,000. Compute depreciation expense for the fourth year. The company uses straight-line depreciation.