Hi,
From your code,
1. It looks like namespace is missing in IService.cs and Service.cs files. Ideally it is a good practice to have a namespace.
2. In web.config file, you are referencing a namespace. This is wrong.
<services>
<service name="WCFREST4.Service">
<endpoint address="" binding="webHttpBinding" contract="WCFREST4.IService"></endpoint>
</service>
</services>
Where is 'WCFREST4'?
3. Service.svc file also has a problem
<%@ ServiceHost Language="C#" Factory="System.ServiceModel.Activation.WebServiceHostFactory" Debug="true" Service="Service" CodeBehind="~/App_Code/Service.cs" %>
If your web.config is having WCFREST4.Service then why is Service="Service" does not have WCFREST4.Service ??
Fix these things and then try to debug first before asking doubts here. You can use WcfTestClient.exe tool for testing the service.