Where do you see the 210, 000, 000 NXT needed for this coming from? If you see that is a potential issue, you must also worry constantly that the top 4 mining pools will collude?
I can't find any charts showing network staking weight or amount of coins staking versus time, but I'd guess that when there is a price spike the amount of coins staking decreases, as people move their coins to exchanges. So, I don't think it's out of the question that a single exchange hack could lead to 51% of staking coins being available to the hacker. This seems to be basically what happened with Vericoin.
I don't think the top four mining pools colluding is as much of an issue as this, especially for ASIC mined coins such as bitcoin and litecoin. At least several of the top ten pools own significant amounts/all of the hardware mining on their pool: KnC, ghash, antpool (?), and possibly others. If any of these pools conspire to attack the network, they are basically conspiring to destroy the value of the hardware that they have invested so much money in, since presumably bitcoin would suffer a huge price drop and may or may not recover.
In the case of someone stealing a bunch of PoS coins and double-spending, their goal is presumably to sell out for bitcoin asap, so there's certainly no similar risk to a hacker who steals a boatload of coins. I think it is probably more likely that a top exchange or several top exchanges could collude to perform a 51% attack on a PoS coin than top mining pools colluding. Bter seems to be the poster child for shady exchanges, but there certainly doesn't seem to be a shortage of them. They are (possibly) risking their reputation, but not risking the value of physical assets (ASIC miners). I put 'possibly' in parentheses since they could pretend that they were hacked and then the "hacker" could perform the 51% attack. Bter seemed to do quite OK for a while after the NXT hack, just the more recent BTC hack that seems to have caused people to abandon them.
I'm not saying that any of these scenarios are very likely, but just that I think the 51% on a well established PoS coin (NXT) is more likely than a 51% attack on a well established PoW coin (BTC or LTC).