Symbol Files bdf newbie questions

The procedure you followed for bdf creation is correct.

Regarding the bus, you need an intermediate device for matching the different bus widths.

A common solution is using a latch for the upper (or lower) 4 bits. Your bus master will then make a double write (4bit + 12bit) whenever it needs to write a 16 bit data on your component. The first write is actually addressed to the latch, while the second one goes directly to your device, in addition to the extended bits previously latched.

IMHO the optimal solution would be integrating this glue logic into your component, so that it exposes your 12 bit interface.