This smells like an incorrect reference in a web.config file.
Can you check your web.configs (you might have multiple in sub directories).
Everything should be referencing v4. Also, verify you have the assembly binding redirect. Like:
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name="System.Web.Mvc" publicKeyToken="31BF3856AD364E35" culture="neutral"/>
<bindingRedirect oldVersion="0.0.0.0-4.0.0.0" newVersion="4.0.0.0"/>
</dependentAssembly>
<dependentAssembly>
<assemblyIdentity name="Newtonsoft.Json" publicKeyToken="30AD4FE6B2A6AEED" culture="neutral"/>
<bindingRedirect oldVersion="0.0.0.0-6.0.0.0" newVersion="6.0.0.0"/>
</dependentAssembly>
<probing privatePath="CMSDependencies\Newtonsoft.Json.6.0.0.0"/>
</assemblyBinding>
</runtime>