Quote - I've long understood what causes the visible seams, which is something I learned when baking textures to a UV map with other software, that you have to extend past the seams slightly.

But why exactly do Poser texture makers use 4000x4000 for example, while the whole rest of the CG industry uses 4096, 2048, and so on up or down the line?

 
Okay, I did some searching and found this: Actually Poser content seems to prefer 4000x4000 maps. This is because Poser version 6 for Mac can only go up to 4000x4000. Other versions can do 4096, so to stay compatible with more versions, content makers use 4000.

Bagginsbill would probably know if this isn't the case though.