You use the group track to create a mix of your original tracks to send to your external effect. Direct a send on each track to the group track to do this, and set the output of the group track to a stereo output on your audio interface (not the same output as your master output!).
Record the result onto a new track. (this track shouldn’t send to the grouptrack or you have built yourself a loop.)
In this fashion you have the original 4 dry tracks playing to the master out and simultaneously sending to the group track, which is processed by your hardware fx. You can use the send levels to add more or less reverb to a specific track, and the fader on the return track to make a balance between dry and wet.
I hope that makes sense 