I have four layers

Question

As in an onion, I have layers.

My first layer is usually calm but has been known to have a few rough edges.

My second layer is hard to read, constantly changing: I can be unpredictable.

My third and my fourth layer share a common interest.

Answer 1

Is it

the Earth?

The four layers are:

the crust, the mantel, the outer core and the inner core.
The crust has rough edges.
The mantel is constantly moving fluid.
And the inner and outer core have the same use.

Answer 2

My guess is:

The Earth. The earth has layers like an onion

My first layer is usually calm but has been known to have a few rough edges.

The crust. Usually calm, except for occasional earthquakes, rough edges could refer to land.

My second layer is hard to read, constantly changing: I can be unpredictable.

The liquid mantle. Very viscous, constantly moves over long time periods and is unpredictable. Causes movement of tectonic plates.

My third and my fourth layer share a common interest.

Inner and outer core. Both very dense layers, made up primarily of iron and nickel.

Answer 3

Another answer -

You are a

Burger :)

Explanation

Layer 1 - Cheese with rough edges
Layer 2 - Fillings/Toppings. Often it's difficult to know what stuff's are included in a burger. They keep changing according to what kind of burger you are eating.
Layer 3 & 4 - Bun which serves same purpose

Here's an example -