There are the Generation4 Base Figures V4 and M4.

There is the Generation5 Base Figure called Genesis.

There are the Generation6 Base Figures called Genesis2-Female and Genesis2-Male.

V5 and M5 are not Base Figures, these are just Morphs/Characters for the Genesis Base.

V6 is not a base figure, just a morph for Genesis2-Female. M6 is not a base figure, just a morph for Genesis2-Male.

As an example, if you buy Victoria 6, you don´t need Victoria 4 or Victoria 5. You only need the Genesis2-Female Base Figure, because its just a Morph/Character for Genesis2-Female. Same thing for Gia, Olympia, Girl6 and Stephanie6 these Characters only require the Genesis2 Female Base!

For Michael 6 you only need the Genesis2-Male Base Figure. You dont need Michael 4 or Michael 5.

As a DAZ Studio 4 User you will find the Genesis Starter Essentials and the Genesis2 Starter Essentials in your Product Library. All three Base Figures are included in DAZ Studio4. 

As I´m writing this , I recognize that it might be really a bit confusing ... :-)