To find the unit rate of discovery, we divide the total number of species discovered by the number of years over which they were discovered.
In this case, 4,000 species were discovered over 2 years:
\[ \text{Unit rate} = \frac{4,000 \text{ species}}{2 \text{ years}} = 2,000 \text{ species/year} \]
So the correct response is:
2,000 species/year