What is the role of R4 in the constant current source circuit

[de Ocampo]

Base current is not highly thermally stable, so not an ideal method.
Do you mean the bases of output power transistors? Higher values than necessary may also cause more distortion.

Q1 Q2 in the image (post #1).
Thanks!

 

[rayma]

The current though R1 (which is approx equal to the output current) is normally set by the Vbe of Q1:
Vbe/R1 = 0.65V/100R = 6.5mA. But with a large R4, the variable Q1 base current will cause a variable
voltage drop across R4, which when added to Vbe, changes the current through R1, and the output current.