iMac 4k with 256GB SSD – Is there an HDD socket?

The SATA port is present on the logic board (you’ll need the cables and rubber drive holder).

You really don’t want to setup a Fusion drive using two SSD drives. Keep in mind a Fusion drive config is similar to a Mirrored drive set so the system only ‘sees’ the one SATA drive and the second PCIe drive is hidden. Which means the second SSD is wasted as the I/O speed of the SATA based SSD will be more often fast enough to read.

Here’s the IFIXIT guide you’ll need to follow: iMac Intel 21.5" Retina 4K Display (2017) Hard Drive Replacement Make sure you use the correct tools and techniques outlined in the guide in removing the display, its very easy to damage! And don’t forget to order the replacement tapes (you might want to get tow sets just in case).

I would just configure the second SSD as your data storage drive and leave the PCIe drive as your boot drive and run your apps from it.

If opening the system is daunting, How about just plugging in a Thunderbolt RAID box. Often times it ends up being the better answer if you want performance.