... R4 and C2 provide the bootstrapping function, which uses positive feedback to increase the input impedance.
Bootstrapping with positive feedback I would not use, a guitar has a complex output impedance and there would be a (tone/pickup) potentiometer setting that causes oscillation.
Old tube guitar amps were a 1MEG input impedance and here 500k would be fine. Lower just shifts the guitar's passive tone control up.
You must have ESD protection diodes at the input jack. I repair a lot of gear that got zapped from the 1/4" phone jack tip.
Why do you think it would oscillate? At low frequencies it just looks like a 1M resistor, in series with an AC coupling capacitor. At higher frequencies, the impedance does drop, but it remains capacitive, just with a lower resistance and the Q would far too low to oscillate with anything, even at 100kHz.
I've simulated the difference in impedance between, the bootstrapped amplifier and a 1M resistor and AC coupling capacitor. There isn't much difference between the two, especially below a couple of kHz. The red is the amplifier's input impedance and the green, is the impedance of a 10nF capacitor (yes, I realised 4.7nF was a bit small), in series with a 1M resistor.
The only potential problem is bad design. If C2 is too small and C1 overly large, the bootstrapping wouldn't work and the input impedance would become R1|R2+R4, but I've mitigated that by making C2 big and C1 small.