Is it possible to connect multiple sources to the HD4 simultaneously?

Yes! All Audioengine powered speakers have multiple audio inputs. Both analog inputs (3.5mm and RCA) are active, so you can connect multiple audio sources at the same time. 

The digital inputs are slightly different, though. Both the Bluetooth and USB inputs share an internal DAC. In this case, the Bluetooth input will take precedence over the USB connection. This means that the Bluetooth input will mute the USB input's audio playback. Simply disconnect your Bluetooth device from the HD4 speakers to resume playback from your USB source device.