Here it is:

If you’re used to doing things “the cs3 way” then you can continue to do so with the classic tween tool. It works the same way as you remember, using key frames as normal, but you cannot use it like a new CS4 tween function or use 3d tweens.

DOWNSAVING:
If you use only classic tweens you will be able to down save your CS4 documents to CS3 without any problem. Using CS4 motion tweens will result in the whole of your tween turning into keyframes as if you had decompiled the swf. (Which might do the trick, or you might start to cry, depending on how you need to use it.)

CS4 Introduces The new Motion Tween. Why Adobe did not use the old name for the Old way and give the New name to the New function I dont know. CS4 Motion tweens operate totally differently than CS3 motion tweens.

The biggest changes that I see are:

• Instead of using a motion tween to bridge two key frames – you create a “span” which is editable in totally new ways. You can click and drag the object at any point on the “span” and the movement is already created. This can be HIGHLY ANNOYING until you learn to use the Motion Editor (which will become your new best friend in CS4). You can click, drag, and scale the length of CS4 animations and it preserves the proportional length of segments. This is huge. [You can also select specific keynote (inner span keyframe) by ctrl clicking on PC this allow you to move it independently in the span.]
• You can copy and paste motions onto new symbols. Snow flake animations [silly example] are a breeze as you can create one and then copy and paste the animation in the same way you can styles in photoshop.
• If you do animation in flash the Bone Tool will simplify your work. Do some research into it if and you’ll be amazed.

I wasn’t sure where Flash was going. Before CS4 came out I thought Flash was dying. Flash has always been in sort of a strange place when it comes to its role in a website. Developers and clients alike wonder if its for eye candy or if you should use is as the technology to build your whole website on and Im not going to argue that here.

The fact is that a lot of the eyecandy can be replicated with JQuery, so why bother learning Flash? Cs4 has changed all that. Now the question is: where to Flash and After Effects meet?

If you still need more info, of if you are a visual learner. Check out this guys video on CS4 and 3d Tweens. I only wish it was on youtube or some other embeddable video site instead of a downloadable video.