For completeness, here’s a couple of bash one-liners. First with head, tail and awk:
restic diff $(restic snapshots --compact | tail -4 | head -2 | awk ‘BEGIN{ORS=" "}; {print $1}’)
Then, if json’s more your thing (from here, but adding a --json flag which I suspect was accidentally missing from the original post):
restic diff $(restic snapshots --compact --json | jq -r ‘.[-2:].id’)