Let the height of the tower be \( h \).
Given angles:
\[
\theta \text{ and } 90^\circ - \theta
\]
Using tan formula:
\[
\tan \theta = \frac{h}{4}, \quad \tan (90^\circ - \theta) = \cot \theta = \frac{h}{9}
\]
\[
\frac{h}{4} \times \frac{h}{9} = 1
\]
\[
\frac{h^2}{36} = 1
\]
\[
h^2 = 36
\]
\[
h = 6
\]
Thus, the height of the tower is \( \mathbf{6} \) m.