Hi

I am designing a 4 layer board (my first design)with all the 4 layers assigned as signal layer. The reason I am going with this stack up is because I want to shield critical signals and I do not want to create a split. The 2nd layer is assigned completely for GND (cu pour) and 3 layer for signal+GND.

I would like to know if this is the correct approach ?

 

Welcome to AAC!

I would like to know if this is the correct approach ?

As a general approach, not likely.

As you mentioned, only critical signals need to be shielded (or routed in a manner that minimizes cross cap). So a better approach is to apply shielding intelligently.

One way to do that is to have every other layer routed primarily in the other direction so you don't have different nets running in parallel for significant distances. Even with a 2-layer board, you can minimize critical signal interaction by spacing out nets around critical signals, minimizing overlap with other nets, etc.

 

For under 100MHz digital signals/20MHz analog on 4 layer board I usually have layer 4 as GND plane and add copper pours as needed around critical signals and connector/signal grounds on the top layer.
For higher speed signal and RF the board design considerations are much more complex.

The green PCB has lots of top layer ground pour because the DB9 signal ports will be connected to equipment with high RFI/EMI background noise. I need to keep that noise energy from the controller and interface chips in the middle.

Blue and red dashed lines are pours.


Blue dashed lines are pours.

https://forum.allaboutcircuits.com/...ocess-layers-in-real-life.194561/post-1831464

 

Usually for a better signal propagation, return paths and ground planes plays important role. If the schematic have any High frequency path, USB differntial pair, High speed signals then we should add a ground plane in the below layer for better signal propagation in design.
I use SIGNAL , GND, VCC, SIGNAL for my embedded system boards
And SIGNAL, GND, GND, SIGNAL for low noise RF and signal boards.
Better if you can go with 6-layer because nowadays manufacturers like JLCPCB lower down it's cost to the same as 4 layer. Moreover provide a wide design innovations.