The GND and Vcc layers are essentially the same (or SHOULD
be) for bypass and shielding considerations, with lots of caveats.
If strip-line impedance is important, it is easier to design and
control with odd layers (i.e., a virtual or real GND between
signal layers), although most boards I have seen use the
middle layer as signal, with large ground planes placed as
necessary for isolation/shielding/separation. On an even-
layered board, it would make more sense to make GND/Vcc
layers 1-3 or 2-4, depending on the enclosure.
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