After I worked around the issue with Lenovo's Yoga Mode Control, my Flex 4 worked fine. But now that it was stable, it was time for me to proceed to the storage drive change I had planned.
The easy part should have been to physically switch the drives, right? Well, if you have ever opened a Lenovo Ideapad Flex 4, you will remember that is not the case with these laptops which are physically anything but flexible.
If you want to replace any part of your Flex 4-1570, sorry, but your weekly nightmare is coming. Unless you are reasonable and just give up.
Replacing anything will require you to remove the whole motherboard case. As Lenovo's Hardware Maintenance Manual shows, opening is possible. After removing 10+ screws, your are supposed to simply lift the case. As discussed in this forum thread, this apparently trivial step is in fact incredibly difficult and costly (both in time and risk of damaging the notebook).
Thanks to the instructions on the forum, I managed to open mine, but it took me an insane amount of time, and I still broke the case.
If you are extremely determined, secure:
I managed by starting with my nails, then using a small (electronic) flat-head screwdriver, and then a regular flat-head screwdriver. In general, your should start at the front and progress towards the back, except the most difficult part is the rear half of the Ethernet port's side. I broke my case when I got to the USB ports there. It was necessary to open the back before opening the rear half of the left side.
Good luck, seriously. And if you read this before buying, buy something else. Unless you love having your up arrow key to the left of the right Shift key (what were your designers thinking, and where in hell were your testers)!