I attended a videographer's presentation about his recent work with the Sony PXW-Z100, which has a slightly larger form factor at about 3x the price. With the theater's very high quality 4k projection system the on-screen image quality was mind-boggling good! One in a while you could see some lens issues towards the edges, but those were very much overwhelmed by the superb color and tonality.
I took a few notes. He felt 4:2:2 was the absolute minimum that was worthy of the format, and his favored bit rate would fill up a 32gb XQD in something less than 10 minutes which seems incredible but I'm pretty sure I got that right! He quipped that he felt he had been given the camera for a token fee compared to what he spent on XQD cards. His rational for buying the prosumer Z100 was to look like a tourist geek, versus a pro suitable for nay-saying and dollar-milking by local authorities.
The AX100E seems to have a much lower data rate. If one is going to deliver 4K, where the message is largely the image quality, the question is how much compromise should be allowed. Shouldn't you just go for it all the way?
Have recently looked at some 4k->1080P reductions on the internet. Maybe that's all the rationale you need for 4K, for the most part those reductions look terrific
. (takes a few seconds for the codec to catch up on that clip).