Are you using 8.2? Check your hotfix level. There is a recent hotfix that addresses the issue of the 4.5.1 radio button being disabled due to 4.6 being installed.
Also be forewarned... .Net 4.6 is not yet fully supported. I just had to remove Visual Studio 2015 RC from my development machine because 4.6 was causing my Kentico install to take about 4 min to load a page on my localhost (I also had to manually uninstall 4.6 and its KB update as well)...not sure how much was totally 4.6 or something else, but once removed it worked fine.