I got this notification
on this question. However, this question did not receive 4 downvotes, as can be seen here
so I'm curious what the notification indicates.
I suppose this specific case was explained satisfactorily in the comments. I will add a brief explanation about an easier way to find the same information in your profile.
When viewing your reputation tab, you can choose: Post, Time, Graph.
After switching to time, the reasons for changes are explained in more detail.
Specifically, in your case after trying this you can see the brief explanations (such as bounty, upvote, downvote, edit) next to reputation changes.
You can find a detailed reputation breakdown here: https://math.stackexchange.com/reputation — but I'd guess that the version shown in the reputation tab is easier to read.
This is not relevant in this instance — but in general, it is good to check the checkbox "show removed posts" at the bottom of your reputation tab, if you're looking there because you're investigating some negative reputation change.