What you can do is have Layer 1, 2, 3 close together. All your fast edges would be contained on the top layer, so that you have access to a reference plane and have the coveted inter-plane capacitance.
And layer 4 would be treated as if it was a single layer board.
You would need to run return path adjacent to each signal.
So you could have ground on the 4th layer if you choose too, but,
it doesn't make sense too if you have a ground already in one of the mid layers.
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