Nope, DDR4-4000 is not "running at 4 GHz". SDRAM 133 ran at 133 MHz, but DDR numbers have always multiplied their frequency by "number of simultaneous transfers" or something like that. (The first DDR multiplied by two - "double data rate".)
DDR3-2133 has a 266.67 MHz memory clock, and a 1066.67 MHz bus clock (x 4), and a "data rate" of 2133 "mega-transfers per second". And a bunch of other details, it's complicated.
The point is, don't take the "4000" literally. It's complicated. And timings and latency can differ between models of the same "data rate".
[background, I'm not sure from your reply whether you already know this]: Usually RAM speed doesn't really affect anything consumers do too much (gaming and video encoding). Also, the tradeoff to RAM speed increasing is also that latency usually increases too, so while you get more bandwidth you usually don't get better latency.
Ryzen is getting improvements here because it's designed as a die containing a pair of 4-core complexes (CCX) that are interconnected with a fabric that runs at RAM speed. Faster RAM => better inter-core performance is the primary mechanism here, not actual bandwidth increases or latency improvements. If you need that stuff you'll want Intel HEDT/Xeons or Threadripper/EPYC with more actual memory channels.
[actual answer]: Second hand but: the replies I'm seeing on Reddit are saying that the performance gains on the interconnect are linear up to 2933 and diminishing up to 3466 and you get little additional gain past there. Full thread[0] and a specific reply from someone who claimed to have tested it[1].
