Can I take Genesis 2 clothes are apply them back to Gen 4 characters?
Yes, it's just as easy/difficult to fit clothing from any generation of character to any other character. All that Marvelous Designer interprets the DAZ clothing as is the exported OBJ file's geometry. And similarly, all DAZ characters imported into Marvelous Designer as avatar shapes to have clothing draped onto are just pure geometry like any other avatar shape.
So for textures you just grab a sample of say denim, then process it to extract Bump/Specularity, and apply that across the board for the clothing?
Yes, exactly. I love detailed texture maps, but for clothing models they are often overtextured to compensate for the model's lack of sculptural realism. Like, the texture of a pair of khakis in real life is pretty much just the weave of the khaki cloth (unless they're really worn out and stained.) over the whole pair of pants. If the pants have interesting and realistic dynamic folds and their OBJ mesh is high resolution enough to show those dynamic details, they should look good with just a khaki material pattern over the whole thing. In the renders I did earlier in this thread, the girl's t-shirt doesn't have any texture maps, it's just a green RGB color with some subtle sss. For the red swimsuit, I just chose a red RGB color, and put some slight turbulence in the bump channel, but again no texture maps.
For things like worn out blue jeans, or a t-shirt with a print on it, you wouldn't want to "import OBJ as garment", since there's no easy way to add the needed texture maps. You'd want to create them as a regular MD4 garment where you can position texture maps on the garment in MD4. Which is a breeze. Just position your texture on the pattern in the pattern window, and it's UV mapped automatically.