The waiting game you are describing is not really about a score. It is about the quiet terror of losing control over how your work is perceived. You did everything right: you responded to the rebuttal, you addressed the concerns, and a reviewer told you they would adjust their score. Then they did not. Meanwhile, another reviewer followed through immediately, and now you are stuck staring at a 4 that might be read as an upgraded 3 or 2, when it was never anything but a 4. That distinction matters, and it is completely reasonable to feel anxious about it.
Here is the practical truth: the action item is not to send a private comment to the AC, and it is certainly not to hint at the oversight in your next rebuttal. Reviewers are human, and they are busy, but they are also professionals. If one of them explicitly told you they would update a score and did not, that is a lapse on their end, not a signal for you to manage their workflow. What you are considering doing, nudging them through a public reply or a private note to the AC, will not read as diligence. It will read as score-chasing. And the AC, who has likely seen this exact scenario play out dozens of times, will not penalize you for a reviewer's inaction. But they might form a quiet impression of an author who is more focused on the number than the substance of the reviews.
Your instinct to protect your paper's outcome is understandable, but you are overestimating how much weight a single point carries in the AC's final decision. The AC has the full picture: the original reviews, the rebuttal, the discussion thread, and the fact that one reviewer moved while the other did not. They are not going to assume the 4 is an upgraded score just because you fear it might be. They are going to see a reviewer who said they would act and did not, and they will factor that into their own judgment. That is not a risk to your paper. That is just the messy reality of peer review. What would actually hurt you is if you let this anxiety push you into a move that makes you look like you are gaming the system.
So here is the concrete move: do nothing. Do not reply to the reviewer's comment with a hint. Do not message the AC. Wait until the deadline, and if the score still has not moved, let it go. The score is a data point, not a verdict. If the paper is strong, it will stand on its own. If it is not, a single point will not save it. Your energy is better spent on the next revision, the next submission, or the next project entirely. The reviewer's silence is not a reflection of your work. It is a reflection of their process. Let it be theirs.