Electronic – 4 layer PCB, mixed signal, multiple different voltages

The term that you're looking for (if you want to find more sources on this) is a split plane. It's used pretty often - you aren't doing anything out of the ordinary.

Just a few things to keep in mind:

• You'll want a separate analog and digital ground on your ground plane.
• Keep the planes aligned over each other - don't put your analog power planes over your digital ground. Check out page 4 of Splitting Planes - For Speed and Power.
• Don't route traces that cross the boundaries of your power/ground powers. See PCB Stack-Up - Part 6. Return Path Discontinuities for some info on this.

You might be able to avoid the finger-shaped power planes if you're clever with component placement - in a pinch, you could try keeping the planes simple and using vias to connect the planes to some wide signal traces for the awkward power pins.

first allow me to clearly state my answer. A four layer board will give better performance. The difference is not stark for all boards, but the four layer board will allow you to route signal, power, and ground, directly over each other in a larger variety of ways. Keeping ground directly under the power plane will reduce crosstalk for close proximity lines and reduce noise by allowing overall better routing choices. With four layers, single point ground is an achievable feat without also cutting the ground plane in two. Essentially, if the two layer solution requires cutting across the ground plane, then you are creating two ground planes connected by two points. This allows a current loop, causing noise, and this can be exacerbated by temperature differentials, nearby electronics, etc. I am not sure as to the use case of this device, but the temperature, nearby EMI, and operating frequencies will all play in. If this is a semi-high speed design, then a four layer will definitely save the integrity of those signal lines. A lot of this comes down to how much resolution you need from those ADCs. If you truly need 16 bits, then I would say a 5% performance increase from a four layer board is worth it.

A great reference which I pull my design guidelines from is... http://www.ti.com/lit/an/szza009/szza009.pdf

Cheers