Can I connect multiple sources to the HD4? Can I stream via Bluetooth and use the USB input at the same time?

You should be able to use multiple sources with the HD4s just fine. The analog inputs on the HD4 are always active. They will play at the same time as the Bluetooth or USB input. Bluetooth and USB share the DAC though, so you can only listen to one at a time, and Bluetooth takes priority. If you're using USB for example, and start using Bluetooth, USB will mute and Bluetooth will take over. If you pause or disconnect from Bluetooth the speakers will automatically switch back to USB after a few seconds.