Basically the memory controller has to work double duty to run 4 sticks instead of 2. Not all memory controllers are fast enough to handle all 4 slots at the full speed. It's going to depend on your specific motherboard though, it's not a global rule that 4 sticks is always slower than 2.

I couldn’t accurately describe the complete technical reason why, but if you have a look at the specs for an AMD CPU like the 9800X3D (https://www.amd.com/en/products/processors/desktops/ryzen/9000-series/amd-ryzen-7-9800x3d.html):

Max Memory Speed

2x1R DDR5-5600

2x2R DDR5-5600

4x1R DDR5-3600

4x2R DDR5-3600

Most samples will actually support frequencies in excess of that, what is published is what is guaranteed to work among non-defective chips.

It has to do with the overall bandwidth of the CPU’s Integrated Memory Controller.

Back before AMD published this info, I had diagnosed a few cases where support for a given frequency changed based on what I thought was capacity (e.g 24GB vs 32GB) but may have actually been related to module count.