On top of that bcache device, I decided to add an LVM layer (making
/dev/bcache* a PV), which is really helpful i.e. with the base problem of
this thread: You can create additional bcache devices and pvmove your data
to the new PV, without interrupting production.

Yes, if you start from scratch with some disk setup, including LVM can
 give you a lot of flexibility and almost always-online.

quoted

I
think I would choose to add bcache to each of the four harddisks,

If you'd do that with a single caching device, you're in for contention. My
gut feeling tells me that running a single bcache backing device/caching
device combo on top of MD-RAID is less straining than running MD-RAID across
a bunch of bcache devices with a common caching device: The MD advantage of
spreading the load across multiple "disks" is countered by accessing a
common SSD.

You are right, if you use MD-RAID, after second thought, bcache on top
of MD is a better choice in most cases as far as I can see. I have
been thinking to use RAID of MD instead of btrfs raid. I tried/tested
it with 3-disk RAID5 for some time, but still decided to use only
btrfs for handling multipe devices.

W.r.t. contention for 1 SSD bcaching 4 HDDs: I can definitely notice
this for some access patterns/tasks. I once chose btrfs raid10 when
all HDD were much older/slower and I wanted high enough transfer rates
on file level. But the HDDs are now all newer/faster model and the
strict need for high transfer rates is gone. It currently makes more
sense to use less SATA ports and also less spinning disks and re-use
HDD(s) for redundancy/backup on a file-system-level.