As far as I understand IP law game rules can not be copyrighted and 4e isnt under any sort of patent so you can do pretty much whatever you please as long as your avoid the trademarks. Heck you could even write a your own "clone" of 4e if you wanted to.
If only it were that simple. You include a tiefling or dragonborn - well, you slipped from "non copyrightable rules" to "copyrighted term". A case could even be made that terms like "armor class" and "hit points" are copyrighted and you would need to swap those out. Heck, even a 4e clone that included "dragonmen", "devilmen", "armor bonus", and "life points"
could lead to a lawsuit if there was even a
perception of trying to confuse consumers into thinking your game is the same as D&D.
Saying "you can do pretty much whatever you please as long as you avoid the trademarks" is pretty dangerous legal advice.
It's a gray area, and probably relies more on how much of a perceived threat you are (not so much as competitor, but as threat to not losing control of their intellectual property) as opposed to what the actual legal precedent is out there. As far as
I understand, there's been more far companies that have caved to legal/financial pressure thinking they can go that route than there have been cases actually going to court to set any sort of precedent. So the real precedent is on the little guys wanting to avoid bankruptcy long before they can ever get a legal verdict. Plus the actual handful cases (if there even are that many) turn on the precise definitions of "mathematical formulas" and "specific expressions of those formulas", so it isn't as simple as "game rules can't be copyrighted, so do what you want".
So, if someone like WotC offers you a free license to use their material under certain restrictions, unless you really, really, really know what you are doing (like the folks at Adamant or Goodman Games), use the license not the "rules can't be copyrighted" route.
That's my advice to anyone, for what it's worth.
As for your actual question of wanting to create new material but be able to share it (as I understand your question to be), I would echo Urizen and have you check out One Bad Egg's
Open Gods project. They have a good grasp on the issue, and it's always nice to just copy a good example and build it into the standard way of doing things than for everyone to each try their own way of sharing.