Extracting narrow-band ZigBee signals(4 MHz) from a wide-band WiFi signal(20 MHz)?
Put band pass filters (5 MHz wide) … Re-sample the chunks to 4 MHz.


Don't do that! If you reduce the sampling rate to 4 MS/s, you need to filter to 4 MHz bandwidth anyway. So you could instead just use 4 MHz wide filters and get rid of the resampling filter.