How R equivalent can be found?
I don't know what you mean by "star delta" connection, but you should start by redrawing the schematic to be more clear:



This is the same circuit, but the resistors have been moved to make it more clear how they are connected. Obviously, R1 and R2 are in parallel, the resistance of R1||R2 is in series with R3, and the whole affair is in parallel with R4.

Solve by simplifying each parallel/series resistor pair to it's effective resistance.