The Wonderboom speakers from Ultimate Ears can be connected to each other via Bluetooth. There are no specific restrictions that only similar models such as Wonderboom 3 & 3 or Wonderboom 4 & 4 can be paired. Theoretically, any two Wonderboom speakers can be paired as long as they have Bluetooth and the appropriate steps are followed. However, there is no specific information on whether a combination of Wonderboom 3 and Wonderboom 4 will work without problems. It is advisable to consult the official Ultimate Ears documentation or test the connection.