Forgot something -

14) copy the folder 'Textures\Men\Items\Shield3\SipahiofthePorte' (containing 7 text files for the shield positions per action) to a new folder 'Textures\Men\Items\Shield4\SipahiofthePorte'

Note: you can also, in step 12), just replace the values in the 3rd line with the new values for your new shield image - in this case you don't then have to do steps 13) and 14)