Are sounds in the SD2 and SD4 mapped in the same locations in the same banks? I'm looking to interchange the two without modifying selection settings on the grill controller on my midi accordion, but I don't know if the addresses and/or control change suffixes are the same.

If you only use the bank 0 1 and 10, the SD2 and SD4 are the same.

SD4 have some more internal and user presets that the SD2 don't have. SD2 is like SD4 in MID mode only.

With the vArranger software, you can add auto-accompaniment features to a SD2 or SD4, playing Ketron and Yamaha styles.

Is the SD2 compatible with Sonar 7 (which I have)? I believe I can create user programs there.

Yes you can, but not easy