Replacing resistors with equivalent resistor
The first calculation you did is correct, R3 and R4 are in series, the combination is in parallel with R2 and when combined it is in series with R1.

The second method of trying to combine R3 and R2 in parallel results in an incorrect answer because they are not in parallel. Devices are in parallel if they have the same voltage across them. The voltage across R2 is split between R4 and R3, thus the voltage across R2 and R3 is not the same, which means they are not parallel.