I think Jaderail gave a thorough answer, but since I’m still new myself I thought it might be helpful to rephrase it, from one noob to another.
What you need to buy depends on what you want to do:
To use V4 and M4 textures on Genesis, you don’t need to buy anything extra. V4 textures work right “out of the box” and M4 textures do also, but in that case you’ll need to go to the Surfaces Tab and change the UV layout from V4 to M4.
If you want Genesis to look like V4 and/or M4—in other words, to have the same basic shape—you’ll need the V4 and M4 Shapes for Genesis. (These are also required for some characters that are described as being “for V4 and Genesis” or “for M4 and Genesis.” Without the shapes you can use these characters, but they won’t look exactly as intended.)
If you want to use V4 Morphs ++ and M4 Morphs ++ on Genesis, you’ll need to own those morph sets, plus Genesis Generation X, plus the V4 and M4 Shapes. Or you could get the Genesis Evolutions Head and Body Morphs; these aren’t identical to Morphs ++ but they are very similar; basically they are the Genesis equivalent of Morphs ++.
If you want to use V4 and M4 characters on Genesis, you’ll need Genesis Generation X, plus the V4 and M4 Shapes.
And you probably know this already, but you can also use V4 and M4 clothing on Genesis without buying anything extra, using AutoFit or other methods. AutoFit works better in DS 4.5. And you can use V4 and M4 hair, but it will likely require some manual adjustments.
Hope this helps…(and someone please correct me if I got anything wrong).