Parts taken from the Amiga 4000 Hardware Guide by Warren Block

   Pin 1: *Reset                  Pin 21: Unused
   Pin 2: Ground                  Pin 22: Ground
   Pin 3: Drive Data 7            Pin 23: *I/O Write
   Pin 4: Drive Data 8            Pin 24: Ground
   Pin 5: Drive Data 6            Pin 25: *I/O Read
   Pin 6: Drive Data 9            Pin 26: Ground
   Pin 7: Drive Data 5            Pin 27: I/O Channel Ready
   Pin 8: Drive Data 10           Pin 28: Unused
   Pin 9: Drive Data 4            Pin 29: Unused
  Pin 10: Drive Data 11           Pin 30: Ground
  Pin 11: Drive Data 3            Pin 31: Interrupt equest
  Pin 12: Drive Data 12           Pin 32: Unused
  Pin 13: Drive Data 2            Pin 33: Disk Address 1
  Pin 14: Drive Data 13           Pin 34: Unused
  Pin 15: Drive Data 1            Pin 35: Disk Address 0
  Pin 16: Drive Data 14           Pin 36: Disk Address 2
  Pin 17: Drive Data 0            Pin 37: *IDE_CS1
  Pin 18: Drive Data 15           Pin 38: *IDE_CS2
  Pin 19: Ground                  Pin 39: *Active (LED driver output.)
  Pin 20: Unused                  Pin 40: Ground

For completeness, the extra four pins used on the mini 44-pin 2.5-inchIDE connector (as found on the A600 and A1200):

Pin 41: +5 volts
Pin 42: +5 volts
Pin 43: Ground
Pin 44: Unused