So DDR4-4266 was a no go in any config - it wouldn't post.
DDR4-4133 is unstable and would only occasionally post.
DDR4-4000 is stable. I have been taking advantage of the Samsung B-DIE memory and tightening the timings at 1.45 VDIMM.
1T at DDR-4000 is a no go, too.
First stability checkpoint below, using IBT 'very high'. Next is to up the uncore, retest and then move on to a Prime95 Blend test. 4.8Ghz uncore fails IBT 2/3 of the way through the test using the below settings, so will work up from 4.5Ghz - I don't want to push the vcore above 1.4v and I've already had to increase to 1.392 to achieve this memory OC.
EDIT: Geekbench (ST: 6593; MT: 21637)
https://browser.geekbench.com/v4/cpu/1845139
EDIT 2: 4.7Ghz Uncore passed IBT @ very high, time for some extended Prime95 blend.