Personally I would do it as follows: -

Layer1: signal
Layer2: 5v plane & 12V track as required.
Layer3: ground
Layer4: signal

The area supplied by 12V will probably be only a small portion of the PCB, 
and I would have 12V to just that area, with the rest of that layer as a 5V 
plane.

However without knowing precisely what you are setting out to do, it is 
difficult to advise you properly. 

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