Hmmm... are you truly saying that you cannot get your 4X to sound like the sample 8X that you played in terms of accordion sounds?
To first make a comparison, please compare straight accordion sounds and make sure you are using the same accordion settings. The 8X has so many embellishments in your video that it is very hard to even hear the accordion part.
I don't know if it is 100% possible, as I do not own a 4X, but I can look at the two PDF files and hear the sounds from other 4X players on YouTube videos. In this case, I would hazard saying that this is more user error than Roland V-Accordion error especially since they use basically the same sound engine with a few minor different name changes.
My suggestion to you is to save yourself the time and that if you want an 8X, play the 8X... if you want the 4X, play the 4X, since you seem to own both.