What I would do to convert 2e material to 4e, is not work from the point of view of conversion, as much as making the best analogues I can using what 4e gives. For instance, taking a 2e Druid and converting it to a 4e Druid of the same level wouldn't work at all - they're entirely different character types. A 2e Druid is really a 4e Cleric (unsurprising, as 2e treats them as a special case of specialty priest) with some powers from the Druid class, but still having an emphasis on healing that the 4e Druid doesn't have.
You'd have to do this in each instance. You'd also need to take a look at various monsters. A lot of them were entirely re-imagined, such as the Lamia, which has nothing to do with the 2e Lamia. You'd really have to build her from scratch using Adventure Tools.
Other than that, thematically they're quite compatible. As a big fan of 2e, I really enjoy what 4e gives me, and I've played through a 2e Dark Sun module in 2008, before it was re-released for 4e.
I'd also strongly recommend using Heroes of the Fallen Lands as a base - the classes are designed to be more traditional, so creating new classes along those lines would go a long way to doing 2e modules in 4e.