4 November 2008: Ami brings Toradora! back from the brink
So it turns out that new character from episode four wasn't Ami—it's Sumire. The two of them just have similar hair.
The real Ami appears in episode five. Her two-faced nature is interesting enough to make me want to watch the next episode, so I won't just be following the show out of general principle.
I rather dislike Ryuuji's slacker mom, although it would be okay if she was an unemployed older sister.