SQL Server 2008 R2 enterprise named instance
Microsoft support Multiple instances on single windows machine so it is fully supported when you want to install another version of SQL Server same machine having other versions. But have you made sure that windows server/machine would perfectly support all four instances. Is the hardware resource optimum for your testing won't you face hardware crunch if all 4 instances are required and they are up and running.

Plus why are you still stuck up with SQL Server 2008 R2 edition. The link you posted in your question is pointing to SQL Server 2008 so are you installing SQL server 2008 or 2008 R2. Both does not falls under category of normal support but yes falls under category of extended support, and that too not for very long and soon will become totally unsupported. I would advise you to do your testing at least on SQL Server 2012 Evaluation edition or SQL Server 2014


  Given that I'm wondering if this should be done same as installing a new named instance or is it an upgrade? 


You can do a normal installation but before moving please read carefully hardware and software requirement for installing SQL Server 2008 R2. If you are installing SQL Server 2008 R2 on windows 8 you must read this article before proceeding