This guy's post seems to tally with the videos on youtube:
"The existing SSD is M.2, not msata which are larger. The M.2 socket supports both PCIe and SATA M2 as per the Service manual which is now on the support website.
I used the much faster Samsung XP941. The XP941 has been replaced by the newer Samsung SM951. Currently both are still available but the XP941 is in short supply and the price has gone up $210 since I bought mine so the SM951 is your best bet."
This guy's post seems to tally with the videos on youtube:
"The existing SSD is M.2, not msata which are larger. The M.2 socket supports both PCIe and SATA M2 as per the Service manual which is now on the support website.
I used the much faster Samsung XP941. The XP941 has been replaced by the newer Samsung SM951. Currently both are still available but the XP941 is in short supply and the price has gone up $210 since I bought mine so the SM951 is your best bet."
https://h30434.www3.hp.com/t5/Notebook-Hardware-and-Upgrade-Questions/SSD-drive-upgrade-HP-spectre-x360-13-4001ns/td-p/5008112
This is what the forum poster is suggesting (single "slot" in connector):
https://www.scan.co.uk/products/512gb-samsung-sm951-m2-(22x80)-pcie-30-(x4)-ahci-ssd-mlc-nand-read-2150mb-s-write-1500mb-s-90k-70k-i
Will this Samsung M.2 work or not?
https://www.amazon.co.uk/Samsung-MZ-V7E1T0BW-V-NAND-Express-Solid/dp/B07CGJNLBB