What's the differences between 2 layer PCB and 4 layer PCB?

I want to design some PCB in elecrow. but do not know how should I need to design 2 layers or 4 layers. it seems there are different price in the page between 2&4layers https://www.elecrow.com/pcb-manufacturing.html 

Multi-layered PCBs are practically preferred because of the intensity of signals they offer. Of all the multi-layered types, 2-layer PCB is the most commonly manufactured as it can accommodate interconnects on its both sides. Depending upon the level of complexity sought after, the layers in PCBs can range from 2 to 8, where every increment means the addition of two additional layers. Thus, you may find 2, 4, 6 and 8 layered PCBs commonly. It is a thing of obvious curiosity to know about the differences between these PCB types. This write-up is devoted to understanding the difference between 2 layer PCB and 4 layer PCB on various grounds.