A concave lens has a focal length of 18 cm.

Step 1. The image. $\dfrac{1}{v} = \dfrac{1}{f} + \dfrac{1}{u} = -\dfrac{1}{18\ \text{cm}} - \dfrac{1}{12\ \text{cm}} = -\dfrac{2 + 3}{36\ \text{cm}} = -\dfrac{5}{36\ \text{cm}}$, so $v = -7.2\ \text{cm}$.

Step 2. The magnification. $m = \dfrac{-7.2\ \text{cm}}{-12\ \text{cm}} = +0.6$.

Step 3. The height. $h' = 0.6 \times 4\ \text{cm} = 2.4\ \text{cm}$.