When a boot entry has boot count enabled (i.e. opensuse-microos-6.15.8-1-default-4+3.conf), then sdbootutil set-default-snapshot 4 doesn't work, as it looks for an entry without the boot count.
localhost:~ # ls /boot/efi/loader/entries
opensuse-microos-6.15.8-1-default-1.conf opensuse-microos-6.15.8-1-default-2.conf opensuse-microos-6.15.8-1-default-3.conf opensuse-microos-6.15.8-1-default-4+3.conf
localhost:~ # sdbootutil set-default-snapshot 4
ERROR: Boot loader entry opensuse-microos-6.15.8-1-default-4.conf not found