PCB Design – 4-Layer PCB Bottom Signal Reference Plane Guide

An advantage of a local power plane is that you can leave all the power routing out of your signal layers and in stead focus on the coupling, routing and impedance control of your signals.

Other than that the best advice is always based on your complete and exact design, so I'll tell you some of my preferences and their reasons, and leave them for you to consider.

For reasons of know-variables I prefer to keep no other layers between the GND and important signals, so in complex designs I try to make as many Signal layers directly next to a ground that fits my stack-budget (of course I'm not spending the money for 16 layers on each design I make!). And if I can only get 1 reliable layer like that, I make sure that layer has only signals and hosts at least the signals that are most important or highest frequency.

For the distances of the stack-up you best call the fab you are having the PCB made at, they know what they can do and what they stock. Once you have those numbers you can use them for your impedance control if you need to.

They can also tell you how accurate their PrePreg procedure is. If it's not very accurate or the layer it is spread on has a lot of copper areas and a lot of gaps as well (this makes PrePreg harder to get uniform) sometimes you will want your Signal and GND on either side of a normal plate, to be able to perform good impedance control. If that is a demand you might want to go for your first choice, but swap the "SIG" and "Sig/Pwr/Gnd" layers.

Another thing you put in your title is Analogue, if you have high-fidelity requirements of analogue signals you are not going to regret splitting your Analogue and Digital power domains completely, including the ground planes and only connecting them at the power-input of your board. You'll be thanking yourself for the extra effort once you find you measure very little digital noise in your analogue signals.

Difference between core and prepreg is that core has cooper filled on both sides of dielectric and prepreg is just full dielectric, no copper on any side of the material.

Since you just sandwiched the prepreg between two cores, which are themselves clad with copper, the prepreg now has copper on both sides of it.

Why wouldn't return current flow on the top side of CORE between SIG-GND since there is a full cooper plane on top side of CORE?

High frequency return currents for layer 1 (SIG) will mostly flow on layer 2 (GND). Just how you want it.

High frequency return current for layer 4 (SIG) will mostly flow on layer 3 (PWR). Making this work well likely requires having good bypassing near all source and load locations for high-speed signals.

Return currents for PWR will indeed flow in the GND layer, because those two layers are separated by just the few mil of prepreg. If there also large ground fills on layer 4 you will also get substantial return currents there, even more so if the core layers are much thinner than the prepreg layer.