To find the ratio of plums to apples, we can use the number of each fruit in the basket.
There are 3 plums and 4 apples. The ratio of plums to apples can be expressed as:
\[ \text{Ratio of plums to apples} = \frac{\text{Number of plums}}{\text{Number of apples}} = \frac{3}{4} \]
Thus, the ratio of plums to apples is 3 to 4.
So, the final answer is: 3 to 4.