You can author UHD Blu-rays on 25GB Single Layer, 50GB Dual Layer, and 100GB Triple Layer discs. 66GB DL and 128GB Quad Layer discs are not available to consumers at this moment.
It appears that 128GB BD-R XL discs have been released by Sony, but I only see them on eBay. e.g. https://www.ebay.com/i/164223683860?chn=ps
Edit: By the way, I found this on AVSForum https://www.diversifiedvideosolutions.com/products.html when I was looking to calibrate my TV, so it looks like at least one independent person has worked out 4K Blu-ray authoring. I’m not sure if he’ll be open to sharing techniques though.