Just looking at the list - is it possible the fourth character is the rom size on the first party titles?
Ahhhhh, I like this!
There is a list at mindrec.com...the old TurboList files...where all the HuCARD's are listed with ROM size...
Just glancing at the list, which confirms your suggestion:
R-TYPE = 4
Raiden = 6
Hahhhahhaha. Hardly conclusive, but I think you are on to something...
FURTHER:
Was Neutopia 2 really 6 Mb?
And I don't think Bonk 3 HuCARD was 8Mb...
But maybe TTi just started getting sloppy...
NECRO, time to compare the lists!