A few thoughts about something in the last scene of SPN 4.21 and it's symbolism.

The shot of Sam breaking the mirror with Dean's face stayed in my head until I finally decided to look up what symbolism is associated with mirrors and the breaking of them. Here's what I found.

This site is talking about mirrors in dreams, but I think it still applies. From Dream Symbol Mirror:
Dreaming that you are looking through a two-way mirror indicates that you are coming face to face with some inner or worldly issue. What you see is related to your persona and unconscious. Seeing images through the mirror may be a safe way for you to consider and/or confront material from your unconscious. Mirrors symbolize the imagination and a link between the conscious and unconscious. Dreaming that you are being watched through a two-way mirror, suggests that you feel you are being scrutinized and criticized.

Both of the guys at the end of the ep are coming face to face with their worst fears. For Dean it's that Sam can't be saved, and for Sam it's that he's a monster. I think that counts as an "inner or worldly issue . . . related to [their] persona and unconscious." And they both feel they are being scrutinized and criticized.

From the same site:
To break a mirror in your dream, suggests that you are breaking an old image of yourself. You may be putting an end to an old habit. . . . Seeing a cracked or broken mirror in your dream represents a poor or distorted self-image. Alternatively, it suggests that you have put an end to your old habits and ways.

I can see both of them having a poor or distorted self image. Sam by viewing himself as a monster deep down,. and Dean by thinking it's his job to save Sam. I do wonder which, if either, of them is going to be putting to an end to that habit. Both?

I saved the best for last. From Yahoo! Answers:
the mirror is a symbol of self knowledge,and revelation because a mirror reflects all phenomena without distinction. it is also a symbol of visual form. by tradition every mirror is also gateway to the Other World like the palace of the demoness Lilith the first wife of Adam.

0.0 (I don't think Syberian is EK, SG, or RS, but the idea did enter my mind!) So does the mirror being broken mean the gateway is gone? Is it foreshadowing that the last seal won't be broken, and that Lilith will die?