At a specifically technical level, that adapter box contained a RF oscillator and a “frequency mixer” - the mixer likely being made of a transistor or 2 being switched on and off by the RF oscillator at a very high speed, with the effect of frequency-shifting the signal from the console. It’s similar to the way a camera’s shutter speed shifts the frequency of things like car wheels, helicopter blades, old tv’s and some LED dimmers to make the frequencies of those things visible to the human eye. Radio systems are almost all built on that concept.

it’s possible that channel 4 just plain didn’t work very well in that design but in my area I’m pretty sure it was just interference. I remember that channel 4 looked empty at first glance but if you sat and watched the snow, it would occasionally pick up some very faint stuff - there was likely enough RF signal there to interfere but not enough for the TV to consistently lock on.