Apparently such a mixed 4/8 bit design is not all that unusual, the most popular 8-bit CPU (Z80) also has a 4-bit ALU: https://www.righto.com/2013/09/the-z-80-

Apparently such a mixed 4/8 bit design is not all that unusual, the most popular 8-bit CPU (Z80) also has a 4-bit ALU: https://www.righto.com/2013/09/the-z-80-has-4-bit-alu-heres-how-it.html https://www.righto.com/2013/09/the-z-80-has-4-bit-alu-heres-...