If $x:4::y:5$, then $x:y$ is:

Given the proportion:
$$x:4::y:5$$

This can be written in fractional form as:

$$\frac{x}{4}=\frac{y}{5}$$

Rearranging the terms to find the ratio $\frac{x}{y}$:

$$\frac{x}{y}=\frac{4}{5}$$

Hence, $x:y=4:5$.