A matched load has reflection coefficient zero. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A matched load has the same impedance as the line
The line and load are both 4 ohms, so the reflection numerator is zero.
Zload−Zline=4−4=0
Zero coefficient means no reflected step
The reflected voltage is 0 volts, so the echo from this boundary is erased.