You can create a 'View' of all the 4 tables which will have selected fields from your 4 tables Employee_Details,Employee_Contact_Details,Department_Wise_Employee and Skill_Wise_Employee.Now create an entity model of your 'View' according to your attributes present in 'View'.For 'View' refer to following sites.
http://odetocode.com/articles/299.aspx[
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http://www.w3resource.com/sql/creating-views/create-view-with-join.php[
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You can write LINQ queries taking into account your entity model class corresponding to created View.Make sure that the View name in database should be same with your model entity class name.Then you can carry out CRUD operations as like as in normal entity models.