I tend to run one of the latest versions of DotNetNuke for my development website, however I still want to compile against an older version so that my assemblies are compatible with the older versions.  So, I create a folder outside of my website (C:\Assemblies) with a directory for each version of DNN (and the Ajax Control Toolkit) that I might want to target.  I fill each directory with the contents of the bin folder from the DNN install (or Ajax Control Toolkit website), and then set a resource path in my project that points to that specific directory.  Voila, running 4.7.0, compiling against 4.5.1.

You'll need to make sure that your project doesn't set the Copy Local property on each reference, since that'll copy the old assembly over the new assembly and break your website.

Hope it helps,