ScreenCalc | Calculate data about any display

I am student and I learn with Unity3D engine. I working on small 2D game for beginning.

I need help with some game logic. Specifically how to generate game object reasonably in 4 columns. In this game I spawn game object that falling down from top of the screen. I calculate 4 columns that cover screen. I spawn randomly game object in this 4 columns. But game play is not good. I need a advice how to do better.

My idea is spawn one or more game object in row and reasonably rotate columns. I already spawn object in row. But how to create logic for switch columns? Something like switch (now first columns, now third, now first and fourth etc.)

There is my spawn methods: