The 4th level artillery Orc Archer in Monster Vault has a 'clustered volley' at will, that is area burst-1 within 20, ATT +9 vs AC, hit d10+6 damage. I'm wondering if this is too powerful; should it be a recharge power, maybe 4-6? His basic ranged attack is ATT +11 (edited) for d10+6 & push 1 square.
I can kinda see this guy as an uber-archer Ranger-style leader of Orc Savages (the Essentials Orc minion-4), but if I used 5 of these guys together and they won init vs a close-grouped level 4 party I think they'd slaughter 'em.
Keep in mind, in your example, it doesn't matter what they recharge on - if they all nuke the party at once, the party is going to be messed up.
I think having it an At-Will is fine - once PCs see him using bursts, they can try and stay spread out to minimize what he can do. If the only worry is nova potential... that could happen with MM1 monsters, too. I remember our party running into issues in Thunderspire Labyrinth with some Duergar who had devestating area attacks - we learned to avoid clustering, and to try and take them down first.
Compare to say... Greenscale Marsh Mystic, from MM1. Level 6 instead of 4, and has Bog Cloud. Burst 2 instead of Burst 1, +9 vs fort is likely much more accurate, 2d8+4 is higher damage, and it dazes. Sure, it is a 5+ recharge instead of At-Will - but if you present the same scenario (a group of them winning init vs a clustered group of PCs), things will end badly for the party.
Honestly, I think the more important thing is to:
1) Not start combats in a way/environment that encourages PCs or monsters to be grouped into a small cluster;
2) Not use encounters consisting of multiple versions of just one monster;
3) Or if you do so, have each monster roll init seperately if you fear this sort of nova potential.