In case most projects inside a WPF app solution target .NET Framework 4, can some class library projects of the same solution target .NET Framework 4 Client Profile?
Is it totally okay, or could there be something wrong with such a combination?
Yes, this is totally OK. Client Profile is a strict subset of the full framework, so the application as a whole will work fine.
The application will require that the full .NET 4.0 be installed, but the class libraries that target the CP themselves can be reused (e.g. as part of another application that also targets the CP).