To find out how many acres of land Kelly uses to grow beets, we need to calculate \( \frac{1}{10} \) of her 4 acres.
\[ \text{Acres of land for beets} = 4 \times \frac{1}{10} = \frac{4}{10} \]
Now we can simplify \( \frac{4}{10} \):
\[ \frac{4}{10} = \frac{2}{5} \]
So, Kelly grows beets on \( \frac{2}{5} \) acres of land.