Sun Inc. purchased a new machine at the beginning of the year at a cost of $500,000. The estimated residual value was $16,000. Assume that the estimated useful life was five years, and the estimated productive life of the machine was 456,000 units. Actual annual production was as follows:

Year	Units
1	82,000
2	83,670
3	87,540
4	91,500
5	93,250

What is the net book value of the machine at the end of year 4 under straight-line depreciation?