The back is not only blurred in the material room but also in the render?

Maybe you have texture filtering on, so it appears blurred? Then use the Python script by SVDL to turn it off (Poser 7 texture filtering off, for P7 and versions above, free here: http://www.renderosity.com/mod/freestuff/index.php?user_id=99327

Are the textures the standard (or simple?) textures by DAZ for M4, because there are also high resolution ones, you find them in Poses under MAT for M4. Try them and see if the problem still exists.