In a 45-45-90 triangle, the legs are of equal length, and the relationship between the legs (let's call each leg \( x \)) and the hypotenuse \( h \) is given by the formula:
\[
h = x\sqrt{2}
\]
Given that the hypotenuse \( h = 4 \) cm, we can set up the equation:
\[
4 = x\sqrt{2}
\]
To solve for \( x \), divide both sides by \( \sqrt{2} \):
\[
x = \frac{4}{\sqrt{2}}
\]
Now, to simplify \( \frac{4}{\sqrt{2}} \), we can multiply the numerator and the denominator by \( \sqrt{2} \):
\[
x = \frac{4\sqrt{2}}{2} = 2\sqrt{2}
\]
Thus, the length of each leg of the triangle is:
\[
\boxed{2\sqrt{2}} \text{ cm}
\]