I changed the way downvote limits are handled. It used to be that -4 was the lowest score a comment could get. Now it is just the lowest score that will be displayed.

I made -4 the lower limit a long time ago because there used to sometimes be a very unpleasant atmos of piling on when someone said something unpopular and the score would go down to -10. But I noticed that some people were starting to take advantage of the fact that this meant you could make an asshole remark at the cost of at most 5 karma points. Now there is no limit to the cost.

The reason you see -5 is that the code that changes the score is just Javascript that runs when you click on the up or down arrow. View source and you'll see.