To transition between a 5:1 and 4:1 stage, you need a special carrier/sun gear that goes from the 5:1 planet spacing to the 4:1 sun gear size. There are actually 5 different carriers for the P60 gear box.
- 4:1 planets to output (required for last stage)
- 5:1 planets to output (required for last stage)
- 4:1 planets to 4:1 sun
- 5:1 planets to 5:1 sun
- 5:1 planets to 4:1 sun
http://banebots.com/c/P60P-CP
Incidentally, if you have mixed 4:1 and 5:1 stages, all the 5:1 stages must appear first in the gear box (nearest the motor), meaning you need to have an input motor with the 5:1 sun gear.