"It depends".

Conforming clothing for one character e.g. V3, will not work very easily with a different character, e.g. V4.  Some of it can be converted using tools like Wardrobe Wizard from www.philc.net.

Dynamic clothing can almost always be made to work reasonably well with little effort.

Conforming figure-based hair can sometimes be made to work with a small amount of effort, and sometimes it takes a whole lot of effort.  Many times the hair can simply be treated as a prop (don't conform it, just parent it to the head) and work quite well.

Prop-based hair can very often be made to work very well with a small amount of effort.

Textures cannot be shared between V3 and V4 at this time, although I've heard about a module in the works for a texture conversion utility over at the DAZ forums.

Shoes and boots can fairly often be shared between figures with a medium amount of effort (hiding the feet, resizing the feet or shoes, special poses for the feet).

Victoria 3 is a very good figure, but has issues with bending, particularly when the figure is heavily morphed.  I feel these problems are much less serious in V4, although V4 does still have some issues with bending, a couple of which are new and weren't ever a major problem in V3.

V3 is a very "mature" figure, in that there is a very, very wide selection of add-ons that are made for that figure (some of which can be shared as above).  There are so many products produced by so many talented people that some truly magnificent items have evolved - significantly, Aery_Soul's clothing and hairs, Blackhearted/Rio's GND2 and associated clothing items, Danae's Manhattan, Tendom/Nike's NT Anna, very large collections of wonderful items by Outoftouch and Pretty3d and too many other masterpieces for any single listing.  Just browsing through the marketplace here or at Daz3d or other sites will give you a feel for how sophisticated the V3 offerings usually are compared to the majority of V4-specific content, much of which has been produced very quickly to take advantage of the recent release of the figure.  

Not that there aren't any great items for V4 yet - to the contrary, Aery_Soul, Val3dArt, Idler168 and many others have produced some early masterpieces - but the market has only been open to V4 content for a couple of months, and V3 has a couple of years of head start.  Prices for V3 content tend to be lower as well, since V4 currently is getting so much attention due to her newness.

Basically think about what you want to do, look for content that you think is worth your money, and make your decision based on that.

PS: if it says Hongyu on it, it's basically awesome, for either figure.

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