Basic nodal analysis question
I think you’re having trouble with how ideal wires work. Ideal wires have a resistance of 0 and all nodes connected purely by wires have the same voltage. In your example, node c is connected to node d, which is ground, so they have the same voltage of 0V.

(I don't understand which node is labelled which in your equations so I'm going to use the letter names)

Your equations become:

(Va-Vb)/R1 + (Vb-Vc)/R3 + (Vb-Vc)/R4 = 0

Va = V1 = 5V

Vc = Vd = 0V

Also since node c and d are the same, R2 and R3 are in parallel which means you can combine into equivalent resistor which may make more complicated problems easier in the future