I asked Brad Wilson about this and it has changed in MVC4 RC.
GlobalConfiguration.Configuration.ServiceResolver has been moved to GlobalConfiguration.Configuration.DependencyResolver
Use this implementation to create a Ninject DependencyResolver for your Web Api:
https://gist.github.com/2417226
In NinjectWebCommon.cs:
// Register Dependencies
RegisterServices(kernel);
// Set Web API Resolver
GlobalConfiguration.Configuration.DependencyResolver = new NinjectDependencyResolver(kernel);