Version 4.5 was designed to be backward compatible to version 4.0 and is indeed an in-place update. Therefore after installing version 4.5 you will no longer have version 4.0 assemblies but you can still target version 4.0 using the backward compatible version 4.5 assemblies.

Nevertheless I have read some worries that some bug fixes (may) break backward compatibility and therefore projects targeting version 4.0 developed on machines with version 4.5 installed may behave differently when run on machines with only version 4.0 installed. If this is a real problem, that is if there are observable changes and therefore version 4.5 is not fully backward compatible to version 4.0, I can not tell.

UPDATE: There is a published list of breaking changes [1].

[1] http://msdn.microsoft.com/en-us/library/hh367887.aspx