Debayering has also become much more sophisticated in the recent past so just because a sensor is mathematically 50% green, 25% Blue and 25% Red does not mean that this is all the usable resolution it has for each channel. We know that with a 5K Epic file we can extract 4K+ worth of measurable luma resolution so even if we use the old 4K luma numbers for the chroma we'd still have 3.6-3.8K worth of chroma resolution. Despite it's irrelevance my guess is that would equate to about 4:3.5:3.5, much closer to 4:4:4 than 4:2:2.
