Yes. All the memory shipped with the 8950 is 4800MHz, but like you pointed out, it will only run at 3600MHz when using four 32Gb sticks. I would consider getting the RAM from Crucial.
Hi there, I searched the forum but didn't see it. Per Dell service manual, if the XPS 8950 goes up to 128GB RAM, the supported configuration of DDR5 memory speed drops to 3600MHz. My question is, can I use 4800MHz Rams? Thanks 
XPS 8950, max memory upgrade