For now, I believe it is best for there to be a functioning Fourth Edition Living World "shortly" after the 4E rulebooks are released. A Living Enworld converted to 4E from 3.5 cannot provide that due to the conversion process - at that time, few people will be equipped to, say, convert the races, the custom spells, or the prestige classes, whatever form they take (or even weapons! Remember that fighters get some powers from weapon choice). We need to understand certain aspects of the rules first, resulting in a lengthy conversion process.
A new Living World might be able to provide gaming almost right off the release of the DMG (the third book this time around), even adventures (We'll have at least Keep of the Shadowfell, and D&DI subscribers probably will have a few Dungeon adventures to run for the characters).
That said - Once 4E is released, the Living Enworld community may well decide that we should make the switch as well, even if at this time it seems that more people resist that step than embrace it. That is something we shouldn't forget, and isn't something that is at odds with a seperate 4E Living World. Maybe some people should look at how some aspects of LEW could be translated to 4E early (and I'm sure some people will, if only out of curiosity). But it isn't something that needs to, or even can, be decided now.