yes, I was told the 4 was a cultural thing in Japan.
The numbering system is fairly logical if you look at the relative capabilities of the mid-range models compared to their top range compatriots, and to the relative capabilities as you move up the generations.
Technics was always the "high end" brand of Panasonic reserved for the quality stuff competing with Sony for example. Thus Panasonic tended to be on the vacuum cleaners and batteries and Technics on the direct drive turntables etc. And why the entry level keyboards ended up Panasonic, yet there was a Panasonic piano too.
Both the numbering and brand name were marketing decisions... but where is ever any logic in marketing?