Usual disclaimer: I am not a lawyer, this is not legal advice, etc.
That's not the issue. If 4e goes OGL and 5e contains any mechanics from 4e, then it will be required to use the OGL license first off, and secondly, it will become defacto OGL simply by using OGL mechanics.
Not at all. 4E contains a ton of mechanics from 3E. The d20 mechanic, the way ability score modifiers are calculated, attack rolls and armor class, hit points, opportunity attacks, move action/standard action, cyclical initiative, abilities defined by a class/race combination, XP for kills, experience levels... et cetera, et cetera.
The thing about the OGL is that it is giving you permission to use the specific language and presentation of the game mechanics. The mechanics themselves are not copyrightable. According to
this article, which matches other writing I've seen on the subject from legal types, it would be legal to clone an existing game down to the last detail, change the names and the wording on everything, and publish.
The value of the OGL is primarily to people who can't pay a lawyer to vet every word of their products and make sure it doesn't cross the line into infringement. That "change the names and the wording on everything" part is critically important. If you screw it up, you can leave yourself open. Furthermore, even if you don't screw up, you could have a big court battle to prove you didn't screw up, and because the US does not follow the "loser pays" model for civil cases, having an ironclad case doesn't mean you won't go broke fighting it.
If you followed the OGL, you have a much better shot at getting the whole thing dismissed quickly, because you have what is essentially a contract spelling out that you are allowed to do certain things. You don't have to prove those rights under copyright law, which is notoriously vague and squirrelly; you can just point to the contract.
Back to 4E and 5E. The question is, if 4E were put under the OGL, what would it mean for 5E? The answer, as I understand it (again, IANAL), is "very little." Wizards isn't going to sue itself for infringement. Anybody who tries to republish 5E and claim their work is protected by 4E's OGL status is still going to be subject to a big court battle to prove it, and they'll still have to get a lawyer to go over their rules with a fine-toothed comb and make sure they didn't stray into using 5E language anywhere. It's basically the same as if they just knocked off 5E and changed the words. You could do it. If you did it right and could afford the litigation, the courts would probably back you up. But how confident are you in your ability to do it right, and can you afford the litigation?
That's not to say they would ever OGL-ify 4E. It seems like a quintessentially un-Hasbro thing to do. But I don't think there are substantial legal obstacles.