Why is voltage drop independent of this 4 Ohm resistor?
If you are familiar with Kirchoff's Voltage Law, then you know that any path that connects your two (ideal) battery nodes (assumed to be at the far left and the far right of your diagram) has the same total voltage across it as the battery. Thus the 2R and the 3R path has the same voltage as the battery. The same is also true for the path containing the 5R and the 1R. The 4R does not enter into this at all, which could be replaced by any other value without changing the result.