It's because 4.5 is simply an update of 4.0, therefore in Plesk the version appears as 4.0 rather than 4.5
The actual long version number (4.0.30319.XXXX) in Plesk, shows it is in fact 4.5 as you can see from the versions list:
http://en.wikipedia.org/wiki/List_of_.NET_Framework_versions
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