One way to calculate an equivalent resistance of a network is to apply a known voltage across it and measure the current drawn by the network. The equivalent resistance of this resistor tree is 4 Ω as it draws 1 A of current from a 4 V source. Let's confirm this. N resistors of N Ω connected in parallel are equivalent to 1 Ω. So each level of the tree behaves as a 1 Ω resistor. Four 1 Ω resistors in series is equivalent to 4 Ω.