Pretty sure we do count it, at least, that's how it was with
Fire Emblem if. Is there a reason why we wouldn't count it if it has songs on it?
edit: rather, it'll be counted as a data disc/DVD at the very least, similar to if's release, as both disc 8 from Fire Emblem if and disc 5 from Fire Emblem Echoes have "DATADISC" written on them, and in addition to extras they do have quite a bit of songs on them as well, so I'd say it should be changed to "4CD + DVD".