Well I don't have the time now to do anything elaborate.
You do realize you're questioning the very veracity of image maps and UVs, right? Perhaps I'm not the one to argue about it with. ;-)

I'll try something more elaborate later. I can paint fairly well when I take the time. ;-)

For now, I did something simple, at 1K, 2K and 4K. The image map used was the same for all three and is 4096x4096.
Rendered in LW, that is. Maybe Poser has other problems. I'll test that out with my more elaborate image I'll do later.
mj41865.home.comcast.net/TESTS/V4_Legs_1K.jpg
mj41865.home.comcast.net/TESTS/V4_Legs_2K.jpg
mj41865.home.comcast.net/TESTS/V4_Legs_4K.jpg

The "pixellation" you see in the small writing is a limitation of the brush. Drives me nuts, but it is what it is. I used Deep Paint 3D, because I remembered Modo sucks for painting across seams.