I'm curious what the OP means by "flexible and open".
For example, it certainly seems easy enough to create new races, feats, powers, items and creatures in 4e -- as easy as in 3.x, easily. Paragon Paths and Epic Destinies should also be pretty easy to create, though possibly harder to balance. How about an entire class? Pretty hard if you're starting from scratch, but maybe not so difficult if you just modify an existing one.
But let's say instead that you want to reintroduce 3.x mechanics like grappling, disarming or breaking weapons, or the AD&D concept of "armor class" (with different values depending on the weapon type used for the attack) -- I could see that being pretty complex.
Alternatively, if the OP likes 4e but hates Encounter powers and wants to remove them completely without destroying balance -- that too could be difficult.
So, I return to my original point: What does the OP mean by "flexible and open"?