Hi, playback heads 4 & 5 were from my perception meant to double the number of echos when both were switched on. It's easy to ascertain which head is being switched by connecting the tape as usual, then removing the tape and then slightly and very carefully just touching the head gap with a flat screwdriver. First with push-pull switch in and then pull it out. You will then know for sure which head is being switched out.
Actually an interesting modification would be to add a switch to the other head so that either playback head could be switched on or off. This would require a bit of circuitry alteration to keep levels consistent, but would never the less add another dimension.