Good question. Seems like they only want to sell the Roku Ultra with the Voice Remote Pro on Roku.com The + versions always came with a better remote. Although right now they have a "Roku Express 4K+" but no "Roku Express 4K" at all. So it doesn't really make much sense.
The way Roku names their products isn't very intelligent. Why is the "Roku Streaming Stick 4K" the only one with "Streaming" in the title when they all stream? Why is the "Roku Ultra" the only 4K device without "4K" in the title?
It would make more sense to name them as follows:
"Roku Express Full HD"
"Roku Express 4K"
"Roku Stick 4K"
"Roku Ultra 4K"
This way every device that is 4K clearly shows it in the title and anything only supporting HD is marked too. That's a lot more consistent and less confusing in my opinion. But what the heck do I know? I'm just a stupid paying customer, not a genius working in the Roku marketing department.