For me it's better calling it the SGL rather than constantly referring to "oh, that book is the 3.0 OGL while that book is the 4.0 OGL".
And no, the SGL doesn't prevent third parties from using the 4E rules. THIS IS the license that allows them to use the 4E rules for the game, it's just not as loose as the original OGL, but probably a step ahead of the d20 license. For me this is good because if they didn't rename it, we'd be having two kinds of OGL out in the market.
Since 4E is derivative of 3E, and 3E had the OGL, someone will be able to replicate the mechanics of 4E and release a product under the OGL that will be somewhat compatible with 4E, regardless of what the GSL says.
What they won't be able to do is use WoTC exclusive IP, or use the d20 Logo.
I'm wondering if part of the drive towards feats with florid names (golden wyverns tail, or whatever) is to associate mechanics with fluff that WoTC can claim as protected IP. In other words, a OGL derived game like Grim Tales could pick up a 4E rules mechanic, but they'd have to change its name because the name is not Open. But that's just a wild guess.
At least, that's my perception of the situation. Would anyone care to correct me?