Decreased DDR5 RAM speed with four sticks. Is it intended?

I recently bought a system with:

- Processor: AMD 7900X3D Processor.
- Motherboard: ASRock x670E Pro RS
- RAM: G.Skill Neo RGB 2x2x16GB 6000MHz CL32 AMD Expo

Testing the RAM speeds I am getting:
- 4800Hz (base DDR5 speed) with 2 sticks
- 6000Hz with 2 sticks and AMD Expo enabled
- 3600Hz with 4 sticks - it doesn't matter if I enable AMD Expo or not.

I made sure I was buying RAM compatible with this motherboard and the AMD Expo combo, yet I'm getting less than base DDR5 speeds when enabling it with four sticks. Is there any limitation I'm not aware of? I couldn't find any on the specification https://www.asrock.com/mb/AMD/X670E%20Pro%20RS/#Specification

If there is a limitation: Can it be overcame? (By, for example, tinkering with OC settings or by a motherboard update).

If it can't be overcame I am still in time to return these sticks and go for a 2x32GB setup. However, I didn't find any 2x32GB RAM listed in the QML with Expo supported :-( https://www.asrock.com/mb/AMD/X670E%20Pro%20RS/#Memory