C4 is a tiny 0.47nF (blue,small like a disk cap). C19 is a little bit bigger and it is 1nF. I can see that one of it's pin is on the ground. Can't see the other pin, but it connect to one of the pot leg (with pot fully on or fully close). My meter would not always beep if it was on the wiper... Anyway... This cap is cutting weirdly the frequency response... I was also wondering what is the function of C20, C22, C15. They connect the output of pin 1 directly to input pin 2 (pin 1 throug R19 for C20). Is it a "negative feedback" for cancellation of the high frequency noise?