As far as I see it, it's just basic business understanding of the payback period. As you see it, 1,000 is remaining at end of the 4th year. In 5th year, the expected net cash flow is 5,000. 5,000/12 ~= 416.67 per month. 1000 / 416.67 ~= 2.5 months.
So you can say, in the first quarter of the 5th year, the payback will be satisfied.