This is how I currently view the depth chart. Alex is the starter until something drastic happens. I think Johnson is doing better and is more polished than Kaep, so in a backup sense, Johnson has the least drop-off from Smith.
I was really high on Kaep when we drafted him, but my enthusiasm is waning. He could still be a great QB, who knows, but the press makes it seem like he's struggling more than the other guys. Plus he doesn't have a large contract, so there's no $$$ bias.
Starter: Alex Smith
Backup: Josh Johnson
3rd String: Colin Kaepernick
4th String: Scott Tolzien