Question: ![schematic](https://i.stack.imgur.com/0Q8kI.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2f0Q8kI.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)

When finding Req between M and N i can't understand why current isn't flowing in R3 and R4.

Answer: 
I redrew your schematic. Do you see it now? By symmetry you can see that the voltage across R3 and R4 will be zero. So, using Ohms law, 0/R = 0.

![schematic](https://i.stack.imgur.com/em6e3.png)

[simulate this circuit](/plugins/schematics?image=http%3a%2f%2fi.stack.imgur.com%2fem6e3.png) – Schematic created using [CircuitLab](https://www.circuitlab.com/)
All the answers are pretty good but just to give a pictorial view of what jonk meant, here's an easy to digest transformation for beginners. 
![enter image description here](https://i.stack.imgur.com/BLZj8.jpg)