He'd almost have to be. He mentioned a long time ago that he was freelancing on a WotC project that he couldn't name due to NDA, and since his name hasn't appeared on anything yet, it has to be a 4e product.
I had always assumed that OotS wouldn't make the shift to 4e (mainly because of Elan), but now I wonder. Elan did change classes (around the same time Burlew was working on the aforementioned product), so now all the OotS members are potentially 4e-legal (depending on how Elan gets re-implemented--warlord, probably. Weird). I imagine Vaarsuvius will be very surprised the first time he discovers he can Fey Step! And aside from him, most of the party will get a significant power boost--Belkar will lose his tiny swords, Roy and Haley will get exploits (though Haley will have to trade her shortbow out), Durkon will get a Wis bonus... and Elan has nowhere to go but up.