WMG / The Cat Lady

The fourth parasite is Susan's ex-boyfriend.

The fourth parasite appears to be some sort of BLAM, because he just kind of randomly shows up with flowers (and a hammer), but: Susan must have recognized him, otherwise she wouldn't have opened the door. He brought flowers like her ex-boyfriend used to send her. Also, he asked her to play the piano, he must have missed hearing her play. And his appearance is what spurs Susan to tell Mitzi about her affair. She chose that moment to tell her because he's the person she was talking about.