No, you are too late for upgrade grace period. If you want CS6 you will have to pay full price or subscribe.
No, there is no upgrade path from CS4 to CS6.
However Adobe is now offering Photoshop CC+Lightroom package for $10/mo if you own CS3 or later. And supposedly this is not a 12month introductory rate, but the regular rental price.
If that is true that would be a steal. Equivalent to over 6 years of rental at same price as ownership, plus you get updates.
It is true. But there is no "price lock" guarantee past one year.
To be clear, $9.99 is not an introductory price. It is the price for those of you who sign up by December 31, 2013.
Reading more of the fine print, I see that this offer only applies to owners of commercial license of stand-alone PS CS3 or higher. If you own Photoshop as part of a Creative Suite or an academic version, you are SOL.
Bad Adobe, bad. You almost had me signing up to your Cloud....