TOPIC: Unidentified risk materializes, query on flow needed

While doing a project status meeting, the project was hit by an unidentified risk that could significantly impact project budget. What should the PM do now?
1) Take insurance to transfer the risk
2) immediately work with the team to setup a risk mitigation plan
3) the first step would be to include the issue in issue log
4) reach out to project stakeholders and inform them immediately

I went with option 3, but according to eduhubspt where this question is posted today as question of the day, the right answer is 4.
Please explain to me why so?

Well I am not a PMP yet, but I can tell you if it was an unidentified risk then it won't be in risk register yet. hence no mitigation plan is available.
Since it impacts the iron triangle (scope, cost, schedule), it has to be immediately communicated to stakeholder.
Question is how it should be communicated and to whom?
Additionally these are realized risk not issues, which makes option 4 a better bet.
I think post risk review meetings, the issues and potential risks will make their way to issue log and risk register respectively (back to identify risk!)

www.pmi.org/learning/library/explaining-...ct-stakeholders-2206