I'd be VERY interested to hear if you (or anyone else on here) has had success with a cheap non-commodore VGA adaptor. Just a plastic affair that links up the various required pins rather than having any ICs in it. I've tried using one with my A4000 and cannot get it to work. If someone has had success with this type of adaptor it might mean my a4k is knackered.
By the way, rather than messing around with modulators for your a4000, why not just buy a scart lead which connects to the RGB port? This will also have sound connectors and will give a better picture than an old modulator.