If not's too much work or confusing, you could offer two versions in some cases.

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I appreciate the response; fortunately, CC3 clothing is compatible with CC4, so there is no need for both versions. I currently have both versions open and can drag content from CC3 directly into CC4 and it loads up normally. It just doesn't work the other way around.

Here is a screenshot > CC4 on Left, CC3 on Right. I dragged a dress over onto the character in CC4 and I can replace the clothing and it works like normal. Same goes for Avatars, skin, pretty much everything I think.
Basically, the downside is that anyone who sells on the marketplace/content store will still have to use CC3 for making content if we want people who don't own CC4 to be able to purchase it.