Its is your assertion that thick hair do not look realistic. It probably depends on many factor, including cultural ones. And there are definitely thin hair for g8F. Look for instance at all the work of HerYun
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Her hair are very thin (and fantastic to use on a crowd because they are so light in terms of polys).
But here are some ways to answer your question (easier to more difficult)
A/ just use the the V4 hair that you like. Hair are generally easy to use between different generation. There are converters, but they are most of the time useless. Just parent the hair to your char, fix the position and it is done.
B/ select the hair that is too thick and look in the surface tab. There may be different surfaces in the hair and you can try to hide some (by modifying the cutout opacity slider).
C/ use the geometry editor to hide some parts of the hair. It is easy (as it does not require any specific knowledge), but can be long and tedious.
1/ select the hair,
2/ (optional, but useful) set the display as wire texture shaded to see the polygons
3/ use the geometry editor (shift-alt-G) and display the "tool settings" tab. Verify that your selection tyoe is "polygons"
In this tab, you see all the surfaces that are in the asset. The purpose is to define a new surface with the unwanted part of the hair in order to hide it.
4/ select polygons on some parts of the hair that you want to hide. There are many tricks to ease this polygon selection and there can be so many polygons in hair that you should use them. You can probably find a good tutorial on the geometry editor. (see for instance
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)
5/ create a new surface with these selected polygons right-click>geometry assignment>create surface from selected and add selected to surface once the surface is created
6/ make this surface invisible (click in the eye on the surface in the tool settings tab or use the surface tab to modify cutout opacity as in B/). You can also delete the polygons (but it is permanent).
7/ continue until you are happy with the result
Note that your are constrained by the existing geometry of the hair and there is no guaranty that you will obtain what you want. But you can try to unthicken hair this way.