In this ckt, does R3 and R4 help in making LED fall times faster (referencing the square wave drive signal)? For argument sake, Q1 and Q2 have identical characteristics. LED's are the same. R1=R2, R3=R4, R3 = 10*R1
Only to the extent that is helps charge the parasitic capacitances in the circuit, which are likely minor.
So it would speed up the LED turn-off slightly.
It would not significantly affect the LED turn-on.
