Turn each UNIT-4 on, then choose W+ Link or Bluetooth® with the Mode button. W+ Link uses the transmitter in the box: X04 with UNIT-4, or X02 with UNIT-4 (Model: MKI).
Hold the middle button for 2 seconds to turn a speaker on. Double press it to see the battery level.
Please follow the steps in How do I pair X04 with UNIT-4?
Please follow the steps in How do I pair X02 with UNIT-4?
Watch UNIT-4 connect to the transmitter:

Watch two UNIT-4 pair over Bluetooth:

UNIT-4 is tuned for studio use. In the AIAIAI app, choose a sound profile or set the 5-band EQ.
Download the app from the App Store or Google Play.