Decoupling of the stepper drives
The stepstick drives are prone to oscillate when driving capacitive loads (long cables) at high current setting.
a 100uF electrolytic capacitor close to the drive can overcome that problem.
Solution: provide a footprint underneath the pololu (between the contact rows) for a 100uF capacitor.
A radial capacitor can be bent 90degrees and placed under the driver (same solution as on ultimaker board).
The 4th axis does not need this (that is close to the power connector and there is no space due to the FFC connector).