Found some links from an old notepad. These are close to a month old, so stability at higher speeds should have improved since then.

ASRock Steel Legend runs at 4800(?) https://old.reddit.com/r/ASRock/comments/xtt3ol/x670e_steel_... His initial 5200 was actually unstable, a comment said he went to 4800, not sure whether 4800 is stable

Gigabyte board could run 128gb "stable" at 5200 (need to expand comment thread near bottom for that) https://old.reddit.com/r/gigabyte/comments/y1ns7j/aorus_x670... Expanded comment about 5200 stable, and link to benchmark with it: https://old.reddit.com/r/gigabyte/comments/y1ns7j/aorus_x670...

TUF X670E can only run 128gb at 4400, not 4800 https://old.reddit.com/r/Amd/comments/ya3gxx/psa_dont_buy_an...

On ROG Strix X670, commenter could run 4x16gb at 6000 (6400 was untable). The commenter previously could only do 5200; BIOS update made 6000 work. OP with 4x32gb was stable at 4400, not 5000, didn't say about 4800. https://old.reddit.com/r/overclocking/comments/xzau6z/x670_m...

Many comments here: https://old.reddit.com/r/Amd/comments/xzj69v/is_anyone_runni...

He's running 128gb at 5200 on a Gigabyte Aorus Master 670E https://old.reddit.com/r/Amd/comments/yggpr5/7950x6900xt128g...

Yeah - I think the key here is 4x32GB specifically, not just 4 sticks. 128GB is much much harder on the memory controller, since those are dual-rank memory modules (or pseudo-4R due to how DDR5 is wired compared to DDR4), and we got 4 of them. So even 4,800 is far from guaranteed in this situation, on any platform atm, and it’s unclear at this point if and when the situation will improve, barring larger memory modules in 2x48Gb and 2x64GB configurations (which would be much easier to run) or the next gen of chips with stronger MCs.