Opening up a MVC4 project in Visual Studio 2010 can sometimes result in a missing System.Web.Optimization errors. You can install the missing packages by using the Package Manager Console from the Tools -> Library Package Manager menu as shown below.
Once the console is open enter
and you should no longer receive a missing reference error. Note that you may need to run Visual Studio as administrator to correctly install the packages.
Another option is to install the pre-release by appending -Pre to the console command:
Install-Package Microsoft.Web.Optimization -Pre