Microsoft are particularly good at making this confusing!
.NET 4.5 is an in-place upgrade to 4.0; you won't see it listed as a separate version in the properties of the AppPool.
If you've installed 4.5 on the server, set your AppPool to "v4.0". There are some settings in the
web.config
file which are required to get your application to use 4.5, but Visual Studio should take care of that for you:
<system.web>
<compilation targetFramework="4.5" />
<httpRuntime targetFramework="4.5" requestValidationMode="4.5" />
</system.web>