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If the board is symmetrical, L2 and L3 are planes, and both L1 and L4 are covered with mask - the impedance is the same.
Impedance is determined by the dielectric in contact with both sides of the trace, the trace width, the trace thickness, and the distance from the trace to the nearest reference plane. For a symmetric board, the impedance on the top is the same as the impedance on the bottom (L1=L4).
In order for impedance to have any meaning with respect to PCB traces, there must be a reference plane or trace in close proximity to the signal trace. If L2 and L3 are not plane layers, then none of the layers have a predictable impedance.
If L2 and L3 are not plane layers then nothing can be said about the relative impedance of any layer.
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