What is the base current of a class ab amplifier with darlington pair configuration
On a single bipolar, wiggling the base by 18milliVolts will cause a 2:1 variation in the emitter (and collector) current. The GM (transconductance) is Iemitter(amps)/0.026volts. At 26milliAmps, the GM is ONE (amp/volts).

For a Darlington, the GM will be affected by resistance shunting the base-emitter of the high-current device. And the frequency (as transient charging current plays a role).

Try a 1Kohm resistor paralleling the EB junction of Q4(high current device), and the same for the PNP device. For more power, consider 100 ohms, or whatever keeps the low-current device in "relative constant current" operation.

At "relative constant current", the low-current devices have no effect on the GM. IMHO