Whenever you upgrade software, it's a good idea to make sure that your system meets the requirements for the newer version. This is particularly important for server applications like Team Foundation Server.
If you're upgrading from TFS 2013 Update 4 to TFS 2015, there is a small difference in system requirements. Make sure that your system meets the Visual Studio Team Foundation Server 2015 RC system requirements:
Server Supported Operating Systems
Client Supported Operating Systems
Hardware Requirements
The old TFS 2013 Update 4 requirements are available at the following location: Description of Visual Studio 2013 Update 4.