post-flash script with naked-provisioner

[WalterPrech]

I flash an image with an custom a/b approach with the naked-provisioner. The image partitions of the flashed device look like this:

Device            Start      End  Sectors  Size Type
/dev/mmcblk0p1    16384  1064959  1048576  512M Microsoft basic data
/dev/mmcblk0p2  1064960 26230783 25165824   12G Linux filesystem
/dev/mmcblk0p3 26230784 27279359  1048576  512M Microsoft basic data
/dev/mmcblk0p4 27279360 44056575 16777216    8G Linux filesystem
/dev/mmcblk0p5 44056576 46153727  2097152    1G Linux filesystem
/dev/mmcblk0p6 46153728 46678015   524288  256M Linux filesystem

I want to install a unique ssh key in e.g. /home/pi/.ssh on /dev/mmcblk0p4.

How to access /dev/mmcblk0p4 in a post-flash script?