In Agile and Scrum, the estimate for a User Story's effort should ideally be a collaborative effort involving the entire development team. Therefore, "Option 4: Team Member - After arriving at a consensus by providing justification" is the most appropriate choice.

Estimates are typically determined through discussion, consensus, and the collective input of the development team members. The team members should provide their estimates and reasoning, and through discussion, they should work towards a consensus estimate. It's important to consider the diverse perspectives and expertise within the team, including the Subject Matter Expert, but the final estimate should be a collective decision.

While the Product Owner's input is valuable, they shouldn't unilaterally dictate the estimate, and the Scrum Master typically facilitates the process but doesn't provide the estimate. The goal is to achieve a consensus estimate that the team believes is reasonable and achievable.