To match the target of 32 terminals (8 in each group) we must insert exactly the missing numbers into each of the four “group” rules.  Starting from

 A8 → 31 29 26 25  
 A6 → 30 15 14 12 10 9  
 A9 → 32 22 18 16  
 A4 → 1 8 7 2  

we need

– Group 1 (A8): 31 29 28 27 26 25 24 23  
– Group 2 (A6): 30 15 14 13 12 11 10 9  
– Group 3 (A9): 32 22 21 20 19 18 17 16  
– Group 4 (A4): 1 8 7 6 5 4 3 2  

This inserts 4+2+4+4=14 terminals, which is minimal.

Updated production rules:
rule_4: A4 -> 1 8 7 6 5 4 3 2
rule_6: A6 -> 30 15 14 13 12 11 10 9
rule_8: A8 -> 31 29 28 27 26 25 24 23
rule_9: A9 -> 32 22 21 20 19 18 17 16