WotC can't stop someone from making a set of OGL rules that mimic 4E. If there is a "Nature" skill in 4E, then there's nothing to stop someone from writing up a new skill called "Nature" under the OGL and - so long as the text isn't identical - giving it the same set of bonuses, making it usable under the same set of conditions, etc.
Even if WotC is aware of this, they have no recourse for stopping it (they can try to take it to court and hope they win via attrition, but that seems extremely doubtful). The only thing that couldn't be used would be protected intellectual properties, such as names of specific characters or names of brand new monsters (e.g. if they have a 4E monster called a "megataur," then someone could make an OGL version that matches all of the statistics, but would have to call it a "mega-minotaur" or something similar).
One way or the other, I think it's only a matter of time before we see an OGL product that's 4E compatible.