Each version of Premiere Elements is an independent program... Adobe will often offer upgrade pricing for a short time after a new version is released, but that is only pricing
Each of the Premiere Elements versions is a standalone version.
You are buying a whole new version with its own serial number. In the Premiere Elements world "upgrade" is an Adobe pricing term only. If you can show proof of purchase of a prior version and you purchase from Adobe direct, you are eligible for the "upgrade" price rather than what it calls it "full" price. Upgrade version and Full version in this context is the same. It is just a price difference.
Sometimes Adobe direct's "upgrade" price may be more or less than what you would pay if you purchase from an authorized reseller who gives its own pricing.
4K is just a resolution. It is the video codec and file extension of the 4K that are just as import to success editing the video in Premiere Elements 12. Version 12 offers project preset and export presets for the 4K size. If history repeats itself, Adobe should be releasing a new version of the program at the end of
September 2014. No one knows the feature set for it.
Whatever your decision, best go with the free 30 day tryout from Adobe before purchase. Keep in mind that once the new version is released, it will be the version offered for tryout not 12.
Please let us know if you have questions or need clarification.