Hello,
1) To run 4 DIMMs above 1800 FCLK requires a fairly good IMC.
2) Mixing memory kits hinders the chances of hitting these speeds even further, as memory kits are binned in the density they are sold in.
3) For best chance of hitting 4K 1:1 at a higher memory density, purchase a 2 DIMM kit with the memory density you require. E.g. 16x2GB.
13900KS / 8000 CAS36 / ROG APEX Z790 / ROG TUF RTX 4090