A sign-only structure factor adds before it is squared. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Signed contributions add before intensity
The signed contributions are plus, minus, plus, plus. Their exact amplitude is 2.
A=1+−1+1+1=2
Intensity squares the amplitude
The intensity is 2 squared, so the exact training intensity is 4.