hong said:
Mearls is/was a senior developer. He isn't a designer.
Not that I know anything about RPG design, but but my impression is that the designers handle the big picture: what is the game about, what themes do we want to address, how does this inform the gameplay experience, etc. If they do write stuff that appears in the books, that would be general background material (who the gods are, what ancient empires used to exist) or how-to stuff (what is an RPG, how to run a game), as opposed to rules and power descriptions. Game developers, like Mearls, are the ones who do the mechanical side of things.
Close, but not quite. Designers write the content, including both mechanics and fluff. Developers work with the mechanics to hone them and make sure they work with all the mechanics done by the other designers (which are often designed at the same time or so recently that not everyone is aware of all of them).
It appears that in 4E, the design team was split into a mechanical side and a story side.
So in answer to the OP's question:
Rob Heinsoo was the lead designer, in charge of the mechanics. A lot of people (probably everyone on the R&D design team) also did design.
James Wyatt was the story lead. Again, he had a lot of help.
Andy Collins is WotC R&D's development manager. He's listed first among the developers in the credits. If follows that he was therefore the lead developer, but I don't know that for a fact.
In the credits, all three of them are listed as the "4E design team," so you could basically say they're the guys that wrote the game. But, again, they had a whole lotta help.
Finally, Mike is credited as a developer, and also as part of the "4E Final Development Strike Team." So it sounds like he had a really instrumental role in making sure the game came together mechanically.