2-4 wire bridging - allows the option for combining the 2-wire and 4- wire audio together on A or B channels. When daisy chaining multiple bases together you can now bridge a 4 wire which allows you to use 4 wire out to send the audio to a mixer, Matrix or other audio source, providing greater flexibility when configuring a system.
Audio bridging provides the the extra functionality where audio coming the external wired PL system is sent to both the wireless users and also the 4wire port on the basestation. Similarly any audio coming from the 4wire into the DX410 base is sent to the wireless uses and also the externally wired PL system 
2W/4W bridging is accomplished via jumper plugs on the base station pcb board which requires the user to remove the base station top cover to access the jumper plugs.