4.0 D&D and Hero Lab
As has already been pointed out, there are still way too many unknowns at this juncture to say anything definitive regarding 4thEd. Assuming that there is a way we CAN support 4thEd under the OGL, we will absolutely do so. Assuming we do so, it would be ideal to integrate 4thEd support into the current d20 data files, having just a toggle setting to switch between them. However, that depends entirely on how much 4thEd differs from the current 3.5.
My best guess is that we WILL be able to support 4thEd under the OGL, but it's NOT likely that one set of data files can support both versions. The reason for this opinion is two-fold.
First, Wizards will alienate a lot of folks if they try to railroad people into DDI, so they will want to compete on what they offer beyond tools like HL. That means that tools like HL will be allowed to support 4thEd under the OGL. Personally, the online design of DDI makes me think that offline tools like HL will continue to have a place in the mix, since some folks will prefer running their tools offline.
Second, Wizards needs to justify why everyone has to switch to 4thEd, which means there must be some substantive changes, else it's going to seem more like a 3.6Ed. Those substantive changes will likely necessitate some fundamental differences in the data file behaviors that will probably preclude one set of data files working for both versions.
At this point, nobody knows anything, so it's all conjecture. If we CAN support both together, we will do so. But it remains to be seen whether that's feasible.
Once Wizards decides to share with everyone what the plan is, we can all start to figure out how we'll adapt. But all indications are that Wizards hasn't even nailed down all the details of the plan internally yet, so that means those of us on the outside won't know for quite some time still. :-(