The Positive Grid BT-4 unfortunately does not support momentary operation at this moments. To setup follow the instructions in:

Is my MIDI controller ‘XYZ’ supported by Quantiloop

The limitation is that hold actions can not be assigned to any of the buttons. This is not a Quantiloop limitation, but requires momentary operation from the controller. As a workaround you could assign a double tap action to the User Button (3) for instance.