Is Bitcoin network an overlay application network? Is Bitcoin network constructed with just Layers 1-4 of the OSI model? Or it's an overlay application network? 

Can someone please explain about structure of Bitcoin network? 

Thank you
It sits in the application layer as there's the Bitcoin P2P protocol that runs on of TCP. The P2P protocol has to be implemented for nodes to communicate with each other (i.e. block, tx, inv messages).