The application to use is substance painter and substance designer. In designer you can make procedural skin and in painter you can use those materials to paint the texture onto your model. Designer has a rather steep learning curve but once you are skilled in it you can make any material you want to. 6000x4000 is actually vey big for textures to find online, if you really want to have detailed skin the best way to do it to make it yourself or ask an expert in designer to make it for you. I can work good in painter but I am not yet good enough, for what I am aiming for, in designer.
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