The fastest validated frequency ever achieved on a DDR4 memory module now stands at 6,666.6MHz, set by Asus ROG and its overclocking partner Bianbao XE. They set the record using an Asus ROG Strix B550-I Gaming motherboard with a Ryzen 7 4700GE processor and a stick of Crucial Ballistix Max memory.
Technically, this is the first time Asus and Bianbao have goosed a stick of RAM to 6,666MHz, as validated on HWBot (via Tom's Hardware). However, they came close enough to call it that a few months ago, when they achieved 6,665.4MHz on an Intel Core i9 10900K Comet Lake system with G.Skill RAM.
This time, Asus and Bianbao hit the mark and went a tad further, and as a result they now hold the top two spots on HWBot's memory frequency leaderboard. There is no mention of the memory timings, but we can assume they were running loose (and obviously fast...very fast).
These kinds of overclocks are mostly academic. The fastest DDR4 memory you can buy is DDR4-5000, by way of Corsair's LPX RAM (G.Skill has also hit DDR4-5000 using air cooling, but has yet to offer a consumer kit at that frequency). And as for Crucial, the fastest memory I can find are two Ballistix Max DDR4-4400 kits.
It's not likely we will see DDR4 memory kits go much higher in consumer realm. At 6,666.6MHz, Bianbo had to use liquid nitrogen to keep things cool. Still, it's interesting to see how far professional overclockers can push these chips, which can be especially helpful when trying to set benchmarking records on an AMD system (generally speaking, Ryzen CPUs benefit more from higher frequency RAM than Intel CPUs).
Now the question is, will anyone be able to achieve 7,000MHz on DDR4, which gets in the domain of DDR5 territory? We'll have to wait and see.