That did cross my mind but, then, strange it would happen only with channel 4.
As you say, it's unlikely that is the culprit but I'll check once I've installed the correct crystal.
From another perspective, I was wondering how it could happen that signal fed into channel 3 shows up for channel 4 as well.
From an analog input perspective, it can't happen, since each channel is independent all the way to the ADC, including.
From a digital point of view, the ADC shouldn't matter (as in, whether it is working or not), since each channel has their own.
So, the problem must be with storing/retrieving the waveform data to/from memory.
Specifically, I'm thinking that maybe one address line might be stuck either high or low and, whenever the memory range for channel 4 is being addressed, it ends up accessing the memory for channel 3.