Yo!
First, you'd need to understand texel density. "8K" does not matter, the choice is made depending on the size of the asset.
To make a long story short: at the average distance you're looking at a character in VAM, 4K is enough.
Now, if you'd do like, stupidly close shots (like for instance macro shots) then you could start thinking about 8K.
But in video games, we generally don't do that, as the cost of a 8K is 4 times bigger than a 4K. We do "detailed" textures which are generally smaller and displayed by the shader the closer you get.
In your case, 8K won't help you make a good looking "look a like". The quality of the texture set is what makes the difference, not it's size. You could have a 8K texture set that looks like ass and won't even be on par with "Kayla" from vanilla VAM.
Your secondary problem (probably) in a look-a-like situation, is that you're probably trying to replicate a character with the morphs but without the texture work. This is not going to work.
It's a challenge even in the movies industry to get a "ressembling" person for CGI/Game avatars. Half the job is done by the mesh, half is done by the texture to get a proper realistic light and shadow matching compared to the real person (the normal map plays a big role).
Your eyes and brain are way too good at noticing "problems"... this is the uncanney valley phenomenon. Bouncing the texture resolution will not fix that issue.
As for the technical aspect, you cannot do transparency. You need to author your own textures. A proper spec set ( color, spec, gloss, normal ).
If you go through patreons or hub's releases of look-a-likes, you'll find extremely good results. And they are not looking like that because of the texture resolution or the lighting (Side note: lighting plays a role at making a character look good. But VAM lighting system is NOT a problem if you do it right). They look like that because those creators know how to create a proper mesh and a proper texture set.
