A battery-operated torch is configured to emit an almost parallel...

For a parallel beam of light, the cross-sectional area of the beam remains constant as it propagates through space.

Illuminance $E$ is defined as the luminous flux $\Phi$ incident per unit area $A$, given by $E = \dfrac{\Phi}{A}$.

Since neither the luminous flux nor the cross-sectional area changes with distance for a parallel beam, the illuminance remains independent of the distance from the source.

Therefore, the illuminance at a distance of $4 \text{ m}$ will be the same as at $2 \text{ m}$, which is $40 \text{ lux}$.