Let's say you spent lots of effort and money buying 50 pieces of clothing for V3.  Will you have to repurchase all new versions of those clothes to fit V4?   And when V5 comes out, will you have to repurchase all new versions again to fit V5?

Victoria 4 doesn't stop Victoria 3 from working, so you can still use everything you have now. In fact, the Morphs++ set for V4 includes a figure which will wear V3 clothes while taking V4 textures and standard head morphs (plus any custom morphs made with V4 to V3 versions), so you have limited ability to use the old clothing with the new figure. But yes, given the amount of work involved it's likely that any conversions from V3 to V4 will cost money - though you could use Clothes Converter 3 (or Wardrobe Wizard if you have Poser as well as D|S) to do a rough job for yourself.

It would be nice, though, to have a dress, as an example, that would automatically fit a tiny distance away from the body surface, so the dress could be put on any figure independently (even a horse).  The computers of today should have no problem calculating such a simple algorithm.  I think this would also simplify the complexity of countless morphs, since the fabric would simply "flow" over any surface.

Just a thought.

Dynamic cloth, which Poser supports and D|S is reportedly going to get in some form, does that - though it isn't quite as simple as that in practice.

Your the V4 to V3 and you can fit all of V3's clothing to V4 :)