welding controller. I am using a 2 sided board for this earlier I had thought of using 4 layer board but then I realised that it would be an overkill for this board
You may be right, it is overkill for the board, but I doubt it would be overkill for this application

A welding spark, probably, creates the hardest environment I can imagine for a control board. You will need to
Have ferrites at the entry to a steel case with your board for all outside connections.
Have all connections to the high amperage optoisolated in a way that does not allow any noise from the high amperage parts enter your steel box.

Using 4 layers will NOT make the above obsolete, but with this environment I would use "belt, suspenders and a piece of rope"

Erik