how to get fsck to work on a LUKS encrypted partition that for some reason will not umount

Question

openSuse 42.3 (KDE) logged in as root

LUKS encrypted /home partition (is /sda4 a primary partition no LVM) is not accessible on boot although unlocked. Apparently a bad block exists.

fsck.ext4  -nf /dev/sda4
e2fsck 1.42.11 (09-Jul-2014)
Warning!  /dev/sda4 is in use.
ext2fs_open2: Bad magic number in super-block
fsck.ext4: Superblock invalid, trying backup blocks...
fsck.ext4: Bad magic number in super-block while trying to open /dev/sda4

then

unmount /dev/sda4
umount: /dev/sda4: not mounted

alas

sudo mke2fs -n /dev/sda4
mke2fs 1.42.11 (09-Jul-2014)
/dev/sda4 contains a crypto_LUKS file system
Proceed anyway? (y,n) y
/dev/sda4 is apparently in use by the system; will not make a filesystem here!

Next step

fdisk -l
Disk /dev/sda: 931.5 GiB, 1000204886016 bytes, 1953525168 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: A-LONG/NUMBER

Device         Start        End    Sectors   Size Type
/dev/sda1       2048    2099199    2097152     1G BIOS boot
/dev/sda2    2099200   18909183   16809984     8G Microsoft basic data
/dev/sda3   18909184  169904117  150994934    72G Microsoft basic data
/dev/sda4  169904128 1953523711 1783619584 850.5G Microsoft basic data

Disk /dev/mapper/A-HDID-part4: 850.5 GiB, 913211129856 bytes, 1783615488 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes

Next try

umount /dev/mapper/A-HDID-part4:
umount: /dev/mapper/A-HDID-part4: not mounted

With fsck again complaining that it s in use. Thus any suggestions how to proceed from here, I mean getting fsck.ext4 to run or how to Potential data loss is not a problem, a full backup exists. (And yes this is ext4 even though fdisk returned Microsoft basic data)

Apart from a browser and the console nothing else is running, df -h also shows nothing. How do I get fsck to start run.