In 3e art Mezzoloths and Nycaloths have 4 arms, in what's more based on the artist misinterpretation that's seemed to carry on with every picture of them. They don't have anything in the stats to support them having that many arms.
But here's the question, do you like them better with 2 or 4 arms anyways?
I generally like them with two arms, as that's the way they've been, but in some cases they'd look neat with a second set of atrophied or vestigial arms.
I'd have to go with KA here....they were two armed in all the 2E art and that's how I want to see 'em in the future. Wasted or rotted vesigial arms would be acceptable, though.
I actually like the 4armed nycaloths.
It seems strange that they'd slap on an extra pair of arms for two creatures without increasing their number of attacks. Should be pretty easy to adjust their stats to give them more attacks like the Thri-Kreen or the multiattack feat like the Xill.
-420
Well - it *was* a bit strange 
... the artist misread his instructions and no one caught it in time (or they felt didn't have the time to redo the art in time for publication). Thus the error was introduced...
I think it was because Nycoloths have the double rake thing when they're flying (they use their footses).
I prefer 2 arms, but since yugoloths tend to experiment on their kind 4 arm versions could also exist. But then stats of monsters should be changed or even template should be invented (multiatack, increased CR, etc...).
Two, because that's how I'm used to them and the stats support it. Although, it would be easy enough to add an extra attack and make them four armed.