To find the unit rate of discovery of marine animals, you divide the total number of species discovered by the number of years over which the discoveries were made.
In this case, 4,000 new species were discovered over 2 years.
\[ \text{Unit Rate} = \frac{\text{Total Species}}{\text{Total Years}} = \frac{4000 \text{ species}}{2 \text{ years}} = 2000 \text{ species per year} \]
Therefore, the unit rate of discovery is 2000 species per year.