With the new strategy of a new series of books each year, and the philosophy of "everything's core," there isn't a need for a 4.5 as such. In a sense, D&D is becoming Call of C'thuhluized.
Here's what I mean, using classes as an example. The core PHB from 2008 has eight classes in it. Supplements and the PHB2, PHB3 and so forth will have more classes, so let's say we have a total of 20 "core" classes by 2010.
At that point, you can re-release the core PHB with the most popular classes that have come out in the last few years. Suppose the Mage Blade (or whatever the class ends up as) is extremely popular, and the Warlord isn't so popular. Release the new PHB with Mage Blade in it, and Warlord out. You can do the same thing for races, spells, feats, rules for grappling, you name it.
What you get, after all of that is new players having the latest material, but the old material isn't really outdated, and the whole line moves forward. If you've played CoC, you know that the game gets a new core rulebook every few years that basically adds or removes some material, changes some of the monsters, adventures and flavor text, and that's about it. You can play CoC with a 20 year old copy of the rules, or you can buy the latest edition...both still work. Even with that, the core rulebook is still an excellent seller, and it becomes the only game in town for a new player.
In addition to that, you can also do something that was talked about with 3x but never implemented: release a core rulebook for a particular product line. Let's say WotC decides to re-launch Eberron in 2010. As a part of that, they can make a custom PHB for it that has Eberron specific art, races, classes, feats ... and so on. Take out the Dragonborn and the Elarin, put Warforged and Shifters back in. Put Artificers in in place of the Warlord. That way a GM can run an Eberron campaign from the ground up. At the end of the year, you're done with Eberron, and can do the same thing with, say, Ravenloft. Spooky gothic PHB anyone?
So no, I don't see the need for a 4.5 edition. I'd say you will see a 5E down the road when they have played out the above formula two or three times.
--Steve
