A04 isn’t being sold anymore, probably because certain arguably core features were never worked out, like HDMI out, and a few other things I’m probably forgetting. The A06 continued to be sold past the A04, but I wouldn’t be surprised if the “out of stock” listing for it on the website is a permanent thing now. It had minor issues compared to the A04, but like other folks I’ve read comments from here, I too have turned my A06 Devterm into a CM4 device about a year ago, and the A06 has been collecting dust in a drawer where I expect it will remain. Personally, I never saw much, if any performance boost on the A06 compared to the CM4. While it had extra cores, it generally ran too hot to use them all. It’s possible I never got my scheduler and gear-shifting settings just right, but it just didn’t seem worth the hassle for an expectedly minor performance boost. Although most code/projects/applications built ok on it, some things had issues. Everything seems to work with the CM4 though, due to the ubiquity of the Raspberry Pi.
Interestingly, the R01 still seems alive on the forums and is still for sale. I never got one, and have been curious and almost got one just to have it and since it’s cheap, but I’m not sure I personally have a use for it. While it’s experimental and underpowered, it also sounds like it’s extremely power efficient, so if all you need is a terminal on the go or minimal graphics/GUI needs, then it might be a nice option for long battery life. I’ve kept an eye on the threads where people are doing interesting things with it, and it sounds like it’s pretty capable for what it is, and especially for the price. But it’s not really something to do anything but the barest minimum of web browsing on, and it wouldn’t be good for watching videos or running emulators or games, either.