We can use the Doppler Effect formula to estimate the frequencies. If we know the difference in frequencies and the speed of sound, we can find the other frequency. Given that the difference is 4 Hz and the speed of sound is 340 m/s, solving for the other frequency gives:
\[
f_2 = f_1 + \Delta f
\]
where \( f_1 = 340 \, {Hz} \) and \( \Delta f = 4 \, {Hz} \). Thus, the second frequency is:
\[
f_2 = 340 + 4 = 375 \, {Hz}
\]
Hence, the correct answer is (d).