XPS 8950, is 4x8GB viable?

In order to give yourself the best chance of success the RAM should be the exact same modules as your original OEM RAM. Not something similar . . . but the exact same part number. The four RAM modules may zonk you down to 4000 MHz.

I agree that having the same modules provides the best chance of success.  It's odd that I can't get a comprehensive response from Dell about whether this will work or not.   A couple of agents said that the 4 X 8GB configuration isn't on the memory specification document so it "probably won't work" but they didn't know if the specification document listed only those configurations that would work based on system design OR if the list is merely some configurations that were recommended.

I had an extended discussion with Crucial Memory this morning and their assessment is that it WOULD work to add 2 X 8GB to the existing configuration as long as the RAM was single rank.   The initial agent wouldn't commit until he had the discussion with a more technical team, but I'm still left wondering about the accuracy and completeness of that memory specification document.

Dell uses proprietary RAM, so consider getting the 2 additional 8GB modules through Dell so that it is exactly the same. The RAM configurations listed in the memory specification document will include only configurations that have been validated by Dell . . . not necessarily all configurations that will work.

Dell doesn't use "proprietary RAM". 

Many users around here get RAM from Crucial because they guarantee it works or they take it back, assuming they recommended the modules for your specific PC model. 

Here's the link to 8 GB Crucial modules for the XPS 8950. And there's a link on that page to order it through Amazon.

trying that photo again

Yes, Crucial RAM will be compatible with the RAM Dell installed. And if for some reason it doesn't work, Crucial will take it back at no cost to the OP.

Besides, look at the ginormous price difference between Crucial and Dell RAM...

I have the 4x8GB setup using OEM Dell ram 4800 MHz for the XPS 8950, however the configuration using 4 slots will indeed take you down to 4000 MHz. Hope that helps. 

@frazierwa If you purchase 2 x 8GB at Crucial it would cost about $80 and 2 x 16 GB would cost about $150. Removing your original 2 x 8GB and installing 2 x 16GB would give you 32GB at 4800 MHz. According to this: https://www.dell.com/en-us/shop/dell-memory-upgrade-8gb-1rx16-ddr5-udimm-4800mhz/apd/ab883073/memory

one 8GB OEM Dell memory module would cost about $130. I am sure you can do the math.

Thanks for the heads-up.  This is clearly mentioned in Dell manual.  The real question is - why spend extra money to "downgrade" to DDR4 performance?