She made a supportive claim in a party meeting, didn't do action about it. Nothing bullshit unless the story is deeper behind public eyes

https://demokraatti.fi/sdpn-120-vuotisjuhlassa-visioitiin-tulevaa-sanna-marin-miksei-4-paivan-tyoviikko-ja-6-tunnin-tyopaiva-voisi-olla-seuraava-tavoite

https://www.is.fi/politiikka/art-2000006208835.html

https://yle.fi/a/74-20119756

For this topic, native or primary sources are better

Also we could deep dive on the "during PM" slice. she said she supports it in Aug 2019, Cabinet forms in December. Believe me (or not) I tried to stay sharp if the topic gains any official action during the time

Edit: this I can't get a link to because twitter but apparently: Finnish Government, using the handle @FinGovernment. January 7, 2020:

"In the Finnish Government´s program there is no mention about 4-day week. Issue is not on the Finnish Government's agenda. PM @marinsanna envisioned idea briefly in a panel discussion last August while she was the Minister of Transport, and there hasn't been any recent activity."

They had to officially declare this because the urban legend was spreading.

After leaving office? Sure, she's said she still supports the idea. Hope this helps