How do you simplify this resistor circuit?
With the diagram redrawn, it becomes apparent that R1 and R2 are in parallel, and that R3 and R4 are in parallel.
So use the formula for parallel resistors to replace R1 and R2 with a single resistor R12.  Do the same with R3 and R4 to calculare R34.
Then you have R12 in series with R34.