Turkish news publication Technopat has published an alleged AMD slide that shows that DDR4-4000 is the sweet spot for AMD's upcoming Ryzen 5000-series(codenamed Vemeer) chips. The origin of the slide is unknown, therefore, we recommend you take the information with a pinch of salt.
The last sentence from the slide reads: "DDR4-4000 is to Ryzen 5000 Series as DDR4-3800 was to AMD Ryzen 3000 Series - good luck!". For the record, the statement is inaccurate since AMD suggested that DDR4-3733 is the performance sweet spot for the existing Ryzen 3000-series (codenamed Matisse) processors. To recapitulate, the Ryzen 3000 parts allow synchronous operation between the Infinity Fabric clock (FCLK) and memory clock (MCLK) up to 1,800 MHz or DDR4-3600. Some really good samples can handle 1,900 MHz (DDR4-3800) before breaking the 1:1 ratio.
If the information is genuine, Ryzen 5000 processors would permit a FCLK up to 2,000 MHz (DDR4-4000). However, it's important to stress that DDR4-3200 is still the official memory speed supported on the Ryzen 5000 processors so it'll still depend on the quality of your chip's integrated memory controller (IMC) that dictates whether you can hit the higher frequencies.
Memory vendors will look to cash in on the Ryzen 5000 processors. Assuming that the improved memory support is accurate, the new Zen 3 processors will surely bolster DDR4-4000 sales. However, it remains to be seen whether they will take advantage of what these memory kits have to offer since going up the frequency ladder typically means sloppy timings.
If we look on the current market, the Patriot Viper 4 Blackout DDR4 16GB (2x8GB), which retails for $84.99, is the cheapest DDR4-4000 memory kit in the 16GB category. The timings aren't as impressive as the frequency though. The memory kit has its timings configured at 19-21-21-41. However, there are some pretty decent DDR4-4000 offerings on the market, but they carry a heavy premium. For example, the G.Skill Ripjaws V DDR4-4000 16GB (2x8GB) boasts timings of 15-16-16-36, but presently retails for $159.99.
AMD will unlock the floodgates for Zen 3 on November 5. There's already word around town that the Ryzen 5000-series processors will land with substantial instructions per cycle (IPC) improvements. Better memory overclocking capability would certainly be the icing on the cake.