One is cost, but it turns out that channel 4 is where the 5th harmonic of the digital video bit clock (13.5MHz) ends up in. Making herringbone interference patterns in channel 4. Channel 3 doesn't have this problem. I installed in a DVD player an RF modulator I salvaged out of a dead VCR, so this is how I know...
A nice feature of this modulator I salvaged out of a Sharp brand VCR is that it will pass TV channels with the power off. Many VCR TV modulators need the power to be on always, and an additional control line to pass or modulate. Which means I don't have to have power on in the DVD player all the time.