Beats

Two sound-producing sources $A$ and $B$ are moving towards and away from a stationary observer with the same speed, respectively. If the frequency of the sound produced by both sources are equal as $400 \mathrm{Hz}$, then speed of sources (approximately) when the observer detects four beats per second is [given, speed of sound $=340 \mathrm{m} {\mathrm{s}}^{-1}$],