Technically I see no reason why someone couldn't use the OGL to construct a 4e clone in the same way people have created AD&D clones.
Largely because what you're doing is the exact opposite.
If you compare 3E with previous editions, you'll discover that the bulk of the difference lies in the core mechanics: The math is swapped around, etc. Most of the ancillary stuff is actually 99% the same. (For example, most of the spells and magic items in 3.0 actually use the exact same verbiage found in AD&D.)
If you compare 4E with 3E, however, you'll discover that the bulk of the difference lies in the ancillary stuff: The core mechanics (attack rolls, etc.) are mostly the same, while occasionally swapping out a d20 roll for a flat +10.
So if you're constructing a retro-clone from 3E, you're mostly just swapping out core mechanics. But if you're creating a 4E-clone from 3E, you're mostly swapping out the ancillary stuff.
And it's specifically that ancillary stuff where copyright can potentially burn you.
The publicly-stated position of WotC (like TSR before them) is that, while you can't copyright game mechanics, you CAN copyright the creative IP those mechanics are describing. This, of course, makes a lot of sense: Just because WotC published a
Star Wars RPG, it doesn't follow that Jedi are up for grabs. Thus, according to WotC, you can't copyright "+2 bonus to AC"; but you absolutely
can copyright the unique IP which is the D&D take on fantasy. And since you can't replicate the mechanics which model that unique IP without simultaneously infringing that IP... well, there you are.
This position has never, AFAIK, actually been tested in court. But there are a lot of reasons to think that it would stand up. (Not least because every single corporation who owns IP that has ever been licensed for a game will be joining in to make sure it does.)
And therein you see the problem: In making the retro-clones, people were redesigning the purely mechanical stuff that you can't copyright. But if you need to redesign the ancillary stuff (which is what a 4E clone would require), then you're going to run headlong into infringing on what WotC considers (and which probably is) legitimate IP.
What you could probably get was something that looks a lot like 4th Edition and plays a lot like 4th Edition
in general, but which has a completely different set of classes; a completely different bestiary; etc. And, ultimately, that's not going to do the job of a clone. (And would probably be nothing more than a fantasy heartbreaker.)