9950X is dual channel, running 4 DIMMs runs them interleaved, with two DIMMs sharing one physical connection, impacting signal integrity severely. AFAIK this has gotten worse with DDR5 to the point that it's generally recommended to avoid 4 DIMMs unless you really can't get enough RAM otherwise. For maximum bandwidth you need to avoid interleaving.

Strix Halo simply has more memory controllers. Threadrippers are also quad channel, and should be able to run 4 DIMMs at rated speeds, but the cheapest Zen 5 Threadripper seems to be almost double the price of a 9950X3D.