What exactly do you want to do with the file ? Do you want to replace a texture for an item ?.
If you just want to replace an existing texture for an item, you open the "ItemTextureEp4.afs" with the afs loader and extract the ".prs" file you want to change.
From that point onward you wont touch the "ItemTextureEp4.afs" again.
You extract the ".dds" file from the ".prs"-file with the "texture manager" and change it with your image-editing software of your choice (Gimp, Paint.net).
Once you changed the ".dds"-file to your liking, put it back into the extracted ".prs"- file and place the edited ".prs" file into the "Psobb/data/items/texture" folder.