White noise of two-sided spectral density $2 \times 10^{-6} \mathrm{~V}^{2} / \mathrm{Hz}$ is applied to a simple $\mathrm{R}-\mathrm{C}$ low pass filter whose $3 \mathrm{~dB}$ cut off frequency is $4 \; \mathrm{kHz}$. Find the mean squared value of the noise output.