Also for spares add in bevel gear sets. Pro 4s are known to ocassionally loose teeth on their front bevel gears when using a oneway. The One-Way brace which comes with the v1.1 and later kits is meant to help with the problem. It is also suggested that you shim the gears properly and that you soak the gears in hot water.
I boil a pot of water pour it in a mug and drop the gears in and leave them till the water goes warm/cold.
There is also an aluminium bevel gear which you can get although i havent tried it yet.