I currently have Lightroom 2. Will I have to buy the full version of Lightroom 4 when it comes out or can I upgrade?
Late last year, Adobe announced that they were changing their upgrade policies for Photoshop so that only people owning the most recent major version would be able to upgrade at a reduced price. This caused a fair bit of consternation among industry heavyweights, and Adobe has recently backed off this position -- but only through 2012.
The moral of the story: Although their current policy allows an upgrade from 1.x to 3.x for Lightroom, I don't know that it's a sure thing that they'll allow 2.x to 4.x. Personally, I'm with Nick - I think it's pretty likely they'll allow it, but if they do, I'd be sure to jump on the upgrade, because Adobe doesn't seem too excited about people skipping major versions.
The current version of Lightroom allows you to upgrade from 1.x or 2.x for the same price, so I would imagine it would be the same with version 4. There are no guarantees, however.
As you may have noticed from the Adobe site, you can indeed upgrade from LR 2.x to 4 at the discount price of $79,00 (same price if you upgrade from LR 1 or 3).
In the Buy screen, choose Upgrade and then I own ... This will apply the discount.