V4 is older than V5. DAZ 3D figures go in generations, and V4 is Gen 4, and thus obviously V5 is gen 5.
As Scorpio says V5 is a character for genesis, and Genesis is the default figure which ships with DS 4. Genesis has the ability to turn into all sorts of different people, with the help of add ons like V5 and M5.
V4 is stand alone. She will only ever be V4, or with the add on packs she can become Aiko 4, Stephanie 4 or Girl4, but these are all paid for add ons. Character packs will also give you more variation in her looks. You can get some free add on characters as well as the large catalogue of paid for characters. You will also need V4 morphs++ to create many of the add on characters. Morphs++ is really an essential add on for V4.
Genesis itself is very versatlie. V5 is just one of the people it can become.