What does R2 have to do with it?
They aren't in parallel because they are connected to the same nodes. Period.
They aren't in series because current that flows in one does not have to flow through the other. Period.
So R1 and R3 are neither in series nor in parallel.
Now, if we look at the perspective of the terminals A and B, we can see that whatever current enters A has to eventually exit B. Since current entering A has no choice but to go through R3 and since any current exiting B has not choice but to go through R4, from the perspective of the resistance as seen between terminals A and B resistors R3 and R4 are in series.
They aren't in parallel because they are connected to the same nodes. Period.
They aren't in series because current that flows in one does not have to flow through the other. Period.
So R1 and R3 are neither in series nor in parallel.
Now, if we look at the perspective of the terminals A and B, we can see that whatever current enters A has to eventually exit B. Since current entering A has no choice but to go through R3 and since any current exiting B has not choice but to go through R4, from the perspective of the resistance as seen between terminals A and B resistors R3 and R4 are in series.
