Good to hear!
For the time being, what I'm doing is creating two asset bundles (a 2x and a 4x), keeping a reference to the two data files in my scene asset bundle.
I'm setting up all the spites with the 2x, and then have a script which checks current platform on Awake() and swapping to the 4x then if appropriate. This seems to be working so far on testing, although it all feels a little awkward.