Thri-kreen attacking with DOUBLE the usual amount of weapons would be broken in 4th ed, as they don't have to worry about the prior editions' limits (where they couldn't use armour and most gear except rings)
Actually, 4E is well-suited to potentially allowing for multiple weapons available to attack with - keep in mind that in order to get multiple attacks you usually need specific powers or such that allow for it. A dual-wielding ranger doesn't get to swing twice with basic attacks - just with his attacks designed for the use of two-weapons. Four weapons woundn't translate to four attacks unless they designed powers for it.
As such, Thri-kreen who could wield weapons in both sets of hands would potentially be quiet doable in 4E. It would mainly just mean that they could have more of a choice of which weapons to attack with at any given time. And maybe there could be the occasional power or paragon path to let them take a couple big swings.
And, I suspect, that may well have been what happened - the initial design of them might have allowed for exactly that. This art could even have been commissioned at the time.... and then become less accurate when, perhaps, playtesting revealed problems with that initial design?
All just theory here, still, but I could see them realizing that while multiple functional limbs didn't look like an issue initially, it could result in problem areas. What happens if the Thri-kreen carries around two 2-handed weapons? Can they use both with Twin Strike? What happens if they want to carry around a 2-handed weapon, a 1-handed weapon, and a shield - can they attack with multiple weapons while gaining a shield benefit? And even if you create more detailed rules to balance this circumstances, you might run into issues with more limbs letting the Thri-kreen hold more weapons/implements/shields with magical properties, potentially getting extra 'slots' of items than other characters.
You could come up with a complex rules set that addresses all this... but by this point, maybe it looks best to just say the extra limbs are non-combat limbs, and useful for the occasional claw strike and quickly adjusting weaponry (thus providing all the primary benefit that one would have gotten out of multiple weapons in the first place.)
Now, at this point, do you redo the art you've already got? Which isn't quite a perfect picture of how the Thri-kreen work, but is relatively close to it, and a pretty good piece of art to begin with? Maybe a half-dozen gamers in existence will find it to be false advertising and be offended by it, but it doesn't seem likely to be a big problem, and certainly not worth throwing out the cover art and having it entirely redone.
Thus, we end up with a picture that implies Thri-kreen can wield several weapons at once even if they can't do so.
Now, is this actually how it happened? No idea at all. Just speculation on my part, on one way it could come about, and what seems one of the most likely possibilities. And I can't really blame WotC, in this situation or a similar one, for not completely redoing the cover art over something that would be a concern to, in the end, likely no more than a handful of people.
