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/)
Imagine applying a voltage from M to N. 

The circuit is symmetrical so there would be no reason for current to flow "sideways" through R3, R4. Hence you can ignore them. The equivalent resistance would be the same if they were shorts or completely open.