Heh, I believe the Roman numeral for 100 is C, and the Roman numeral for 500 is D! This is a bug in the way the instrument names are being handled when they are loaded: Dorico is interpreting those letters as Roman numerals. This is on our list to be fixed, but I’m afraid it won’t be fixed in the forthcoming 1.0.30 update.
I think your basic approach is sound, but perhaps you can use Part 1, 2, 3, and 4 for now instead of A, B, C, and D!