The codes for each instrument always take the form letter-number-letter-number, and can be derived as follows:
1. The first letter of the instrument's country of origin (in English).
2. The number of letters in that country's shortform English name.
3. The first letter of the instrument itself.
4. The number of letters in the instrument's name.
This can be seen in the codes already known:
Ektara I5E6 (origin: India)
Gunjac C7G6 (origin: Croatia)
Shamisen J5S8 (origin: Japan)
Cavaquinho P8C10 (origin: Portugal)
With this in mind, the gaps can now be filled as follows:
1. Masenqo E8M7 (origin: Ethiopia)
2. Erhu C5E4 (origin: China)
3. Balalaika R6B9 (origin: Russia)
4. Choghur or Chagane A10C7 (origin: Azerbaijan)