Write \$V_{A}\$ in terms of \$V_{in}\$ and the resistor values algebraically in a Op-Amp
Since the op-amp inputs are (ideally) open circuits, case 1 is correct.

Case 2 is wrong conceptually.  In the actual circuit, resistors \$R_3\$ and \$R_4\$ are series connected - all of the current through \$R_4\$ is through \$R_3\$.

However, in your case 2 schematic, this is not the case; there can be a non-zero current through the wire connected the two resistor branches.

Consider the following instead:







simulate this circuit &ndash; Schematic created using CircuitLab

Now, this gives the same result as case 1.