At least in my country (New Zealand) only Newegg (which ships from US and has very expensive shipping) seems to have these 4 stick DDR5 kits and only from Crucial and they are all more expensive than a 2 stick kit with the same capacity, so I'm not sure why anyone would get them.
Populating all of your RAM slots with matched RAM improves your overall bandwidth. A 2x8GB set will run in dual-channel mode, the 4x4 set will run in quad-channel mode (if the CPU supports it, most times it's dual-channel.) The 4x4 set will allow for more bandwidth vs the 2x8. Not quite double, but it reaches at it. Every other metric doesn't really see much of an improvement, though, so this only helps out in bandwidth-intensive applications.
In my (and probably most peoples') case I'm running in dual channel, so that's two 16GB DIMMs per channel. This makes the memory controller clock down to 4000MHz rated.
You can: https://www.newegg.com/corsair-64gb/p/N82E16820236933
Maybe once prices come down more I'll consider going to two sticks of 32GB instead of four sticks of 16GB, who knows.