Changing the naming scheme and jumping to 5000 is just unnecessary. I've said this before, the Ryzen naming scheme is not confusing to the marketplace. It is only enthusiasts and techies who even know what "Zen" is and means, and even then it's really not hard to keep track of which Zen is in which Ryzen.
Why does it matter which Zen is in the APUs and which Zen is in the desktop CPUs? It doesn't. The 4000 APUs may well be Zen 2, but they are still the first products of a new Ryzen series, and therefore get the 4000 moniker. The same with 2000 and 3000 APUs. The fact that there's G, H, and U in the names clearly differentiates them from the no-letter and X models which are desktop CPUs. Even the numbers make sense for progressively higher performance tiers. Yes, it might be take a moment to realise why a 4600X would outperform a 4700H when the number is lower, but the H shows it's an APU and inherently you'd expect a bit lower performance. Plus, the APUs aren't desktop chips, so the consumer is not going to get confused by the comparative performance because you can't get H and U for desktop.
If there's going to be any name change then the x900 parts should be exclusive to Threadripper, just like they always were. Hell, it would be an interesting exercise in mindshare and public perception. Make the 4800X 16 cores, 4700X 12 cores, 4600X 8 cores and introduce a new 4500X at 6 cores. Suddenly it looks like AMD have increased core counts across their entire range from the previous generation. "Wow, AMD have doubled the number of cores from the 3800X! Good guy, AMD!"