I have a Castle Mamba Max Pro and a Castle 1410. The owners manual says that the wire needs to be a certain length from one to the other but the dude I purchased it from really did a crappy splice job. I want to cut out the old section and resolder. What is the designated length supposed to do? Will I have any problems being 4cm shorter on the wire?

If the wire reaches the motor from the esc without being tight your good to go, shorter wire = less weight, just make sure it has a little flex in it so when your chassis flexes you dont pull the wire from the esc or motor.

If the motor and esc are seneorless, than the motor wires must be the exact same length. It has something to do with the signal from the esc to the motor.
Check with Castle before doing anything.