Hi! I am trying to understand your email. What is CGROM (OA) and/or (OB)? Could you explain a little bit more?
I am using several types of LCD and VFD displays. Sometimes you can switch between 2 CGROM using hardware switch (jumper which needs to be open or close). Usually LCD has one built-in CGROM and you can define your own characters and if you want diffrent character set (i.e. Japanese) you have to order LCD display with this character set.
4 bit mode or 8 bit mode - it doesn't matter. It is working the same if you want to display the characters only.

Jacek