First, we need to find the input resistance $R_{in}$. For a common gate (CG) amplifier, the input resistance is given by:
$$R_{in} = \frac{1}{g_m}$$
Plugging in the given value for $g_m$, we get:
$$R_{in} = \frac{1}{4 \, \text{mA}/\text{V}} = 250 \, \Omega$$
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