Reduce filesize of 4000 images

[fmw42]

Put the filter before -resize. Settings come before operators. Use a better filter, such as lanczos or one of the others or take the default. But why are you posterizing. That may be the cause of your issue. You could also use -thumbnail rather than -resize, since it will do an automatic -strip.

See
http://www.imagemagick.org/Usage/resize/
http://www.imagemagick.org/Usage/filter/
http://www.imagemagick.org/Usage/filter/nicolas/