Nothing. You're still free to use WCF where it is most suitable, or at your own discretion.
ASP.NET MVC has supported a RESTful communication style since its inception, and many people use it as a thin veneer for RESTful services. That doesn't automatically cause WCF to go obsolete, or make ASP.NET MVC the One Tool to Rule Them All™.
This is why carpenters and other craftsmen don't just have one type of hammer. They have several different types, each optimized for a particular type of hammering.
To help you decide which to use, listen to this Hanselman podcast:
This is not your father's WCF - All about the WebAPI with Glenn Block
How does WCF fit into a world of Web 2.0 lightweight APIs? What's the
WCF WebAPI and how does compare to services in ASP.NET MVC?
I haven't personally looked at it yet, but it wouldn't surprise me if the Web API you refer to in ASP.NET MVC 4, and the new WebAPI in WCF, turn out to be the same thing. Phil Haack is probably using WCF to implement WebAPI internally in ASP.NET MVC 4, or they both resolve to the same internal mechanism.