You can instruct mono to launch a particular runtime version with the --runtime
flag. In this case, it would be this way:
#!/bin/sh
exec /usr/bin/mono $MONO_OPTIONS --runtime=v4.5 "/usr/lib/mono/4.5/xsp4.exe" "$@"
However, as far as I understand, the 4.5 profile of .NET doesn't include changes in the runtime, but in the class libraries and compilers. So if the above doesn't work, it simply means that the version of the runtime that one needs to use to run a 4.5 application is simply the 4.0.30319 version.
The best way to understand this is to include this little image diagram grabbed from a Scott Hanselman's blog post: