I was wrong: so Q4 is relevant in the circuit? Is not some kind of buffer.. I guess..
Of course it's a buffer..

(but a misbiased one here..)
An
ideal buffer should have its Emitter biased at exactly half of power supply voltage for equal swinging output signal..
A real word one should have its Emitter biased higher than half of power supply due to Emitter and Load resistors voltage dividing effect when sinking current..
(actually, neither its sourcing current capability is ideal but this isn't our point here..)
A full wave rectified signal has to be undistorted (or symmetrically distorted, at least..) to be perceived as octave effect..
(it already suffers enough from its inherent crossover distortion "pathogeny" due to diode(s) forward voltage drop..)