PCB 4 Layer StackUp 2 Questions
Here are some general pointers since all we know at this point is that you are driving some high currents (12A) from a battery pack:


The ground and power planes are a good idea when there are going to be many chips / devices connected between them, distributed across the board. The planes set up a sort of distributed capacitance and low inductance which is effective at high frequencies. It is not a good idea to have the power for your motors flowing through the plane. The width of the trace and copper thickness determine the conductivity and total copper loss in the trace: outer layers can dissipate more due to convection.
Identify your high current paths. This will typically be from the battery +ve terminal to a MOSFET/switch, to the motor, and back. You should try to keep this path length as short as possible: perhaps even using wires instead of PCB traces. 
You also mention that you need to draw only one 0.5mm trace line for 1.8v from layer 3 which I suspect is your control line. In this case you should lay it very carefully to ensure that the motor currents do not cause ground bounce.


In summary, it would be better if you were to post your circuit (some abstraction should be ok) in addition to how you plan to lay it out; and ask for feedback.