Yes, four modules are quite bad with DDR5. I explain this in detail in my
RAM thread under 2).
You will see that there are no four-module RAM kits for DDR5 at all, and that's for a reason. Once you use four modules on any Z690 DDR5 board (also from other brands), the frequency that the memory system can reach suddenly drops down considerably, as you see in my thread. They might be able to improve the situation somewhat with a new BIOS, but i don't think it's possible to do so in a way that would allow anywhere close to DDR5-6000. I would say that with the current generation of DDR5 board designs, with four modules, it's more likely that they will always stay way below. With DDR5 it's always highly advised to use two modules only (and even with DDR4 that is advised, but four modules can at least work decently there).
DDR
4 is actually better for high-capacity/high-density memory configurations, because:
- It doesn't experience such a drop in the electrical properties of the memory system when using four modules
- Big DDR4 modules and kits of four are readily available for a much lower price
- Four-module kits are actually certified on the QVL
- They will most likely outperform their DDR5 equivalent due to DDR4's lower latencies, vs. DDR5's necessary low frequencies at this configuration (meaning, the fact that the higher DDR5 latencies can't be compensated by higher frequencies anymore, because four modules force lower frequencies to be stable).
As for the performance scaling, read here,
https://www.techpowerup.com/review/ddr5-memory-performance-scaling/
So, the MSI specification is not to be taken lightly. Even looking at two modules only, DDR5-6200/6400 kits are really causing some problems on boards with four RAM slots. That's on the upper end of what the signal quality currently allows for on DDR5, unless you pump in ungodly amounts of voltage or use other tricks that are not the best for a daily system. Watch
this spot in the video here where buildzoid talks about that issue. DDR5-6400+ might only work properly on boards with two RAM slots, because the signal quality is better there than a four-slot board with two empty slots. And it also depends on the CPU's IMC (memory controller), some might get unstable at DDR5-6400, in that case, you have to try manually setting the DRAM frequency to DDR5-6200 or lower. But with four modules, you cannot dream of such RAM frequencies.