how can you adress hundreds of I/O with only 4 ports and without a bus
On one occasion I had 400 < 500 (do not remember exact count) I/Os using 8 CPLDs (the old Intel chips they sold to - was it Atmel or Altera). On another I had 48 using IIC.

For A PLC I would use IIC (plug and play) and, if that was too slow @ 400 kHz I would use (again) a CPLD.

Erik