Fantasy Concepts uses a lot of the Saga-esque mechanics derived from OGC. Instead of daily powers, FC has powers that are activated by Action Points. In terms of creating an
OGL 4E, you could use many of the same sources that Fantasy Concepts used, and it would not be hard at all to derive a
4E-esque
OGL game.
However, the question remains, as always: while you can do it, why would you want to? Is it just for the challenge? Or do you have other plans in mind?
With Regards,
Flynn