.Net Framework Compatibility issue

Question

I have an application with kind of following structure 

 

1. Class Library (Built with .Net Framework 4.5) 

2. Web API (Built with .Net Framework 4.5) 

3. Console Application (Built with .Net Framework 4.0)

4. Web Application (Built with .Net Framework 4.5)

 

My requirement is  to consume Class Library method in my Console and Web Application, but due to Compatibility issue I can't not refer that class library to my projects. 

I found a solution that consume this Class library in WEB API project and from my Console and web application post a HTTP request to WEB API. 

one way it will work, but I want to be make sure that each time Web API should run and return true result. 

Please suggest if there any other safe approach to do this operation, without changing the framework of any project