4дм:2

It seems like you're expressing a ratio or division involving "4 дм" (which likely means 4 decimeters) and "2". To simplify this, you can divide the two quantities:

\[ \frac{4 \text{ дм}}{2} = 2 \text{ дм} \]

So, 4 decimeters divided by 2 equals 2 decimeters. If you meant something different, please provide more context!