On the 522:

Hold “Play” when switching on the unit to set it in sync slave mode (if it isn’t already).

On the A4:

Open “MIDI config” in the global menu.
Open “MIDI port config”
Set “Out port func” to MIDI
Set “Ouput to” to include MIDI

Then go back to the “MIDI config” menu
Open “MIDI sync”
Tick the box “clock send”
Tick the box “transport send”
Make sure the receive boxes are un-ticked

Now the 522 will sync to the A4 tempo, and will play and stop according to the A4. If it doesn’t, restart the 522 while holding “Play”.

To hear the 522 sound through the A4, go to the FX track. Press the “ext in” button (osc 1). Adjust volume, panning and effects for the two input channels.