If you can afford it then for Intel get a LGA 2066 CPU like the i7-9800X or the i7-7820X that both have a street price not much higher than the 9900K. And you would get QuadChannel DDR4 support… 32GB RAM is the bare minimum to really work with UE4 with no issues or slowdowns.
For AMD get a Threadripper that has QuadChannel DDR4 support like the 1920X