For a 4 layers PCB (with ground plane), covering with ground polygon on Layer 1&4 necessary?

It is a good practice to provide Ground Planes into the Top layer, and Bottom Layer. If you can't accommodate in the Top/Bottom layers, Make sure that you make a Top-Bottom Via which connects in the internal plane. This reduces the current return paths. Generally Via's are placed on the corner and wherever free space is available.

TI has great design guidelines. https://www.ti.com/lit/an/szza009/szza009.pdf