"An Arduino is only capable of really crap sound on a single channel.An Arduino is only capable of really crap sound on a single channel."
Not true, you can digitize 16-bit single channel mono audio at 44.1KHz and store to SD card with a 16 MHz Arduino.
Take a looke here:
http://forum.arduino.cc/index.php?topic=180769.0
You need an external SPI ADC and it will use all the capacity of the Arduino.
The other request, 4 channels at once: you can sample 4 channels simultaneously with the proper ADC, but you are still limited internally by the uC's databus to moving the data from any one channel at a time. A fast enough processor will let you move all 4 channels in the same time period to appear as if all were being done at the same time.