In addition to what Dan Hulme wrote, I'd like to add that nexus devices have sickstock firmware images for previous versions of androidAndroid, which can be easily installed. You can downgrade your galaxy nexusGalaxy Nexus to ICS, or your nexusNexus S to gingerbreadGingerbread. The nexusNexus 4 shipped with 4.2 but soon yours will have 4.4. You will be able to load older versions later on if that kind of testing is important to you.
I found this to be very valuable when testing with the nexusNexus S because it went from 2.3 to 4.1. You can even rely on cyanogenmod to extend the life of the phone as a test platform, if you trust that cyanogen is sufficiently close to stock for app compatibility purposes.