Try grouping the four terms by what they actually measure. Saturation capacity, field capacity and permanent wilting percentage all answer the same question: how much water, as a percentage or fraction, does the soil hold at a defined state. These are the classical soil moisture constants used to plan irrigation schedules.
Evapo-transpiration answers a different question: how fast is water leaving the soil-plant system. It is a flux, expressed as depth per unit time (mm/day), not a fixed water content value.
Since three of the four terms are moisture contents and one is a rate, the odd one out is clear.
\[\boxed{\text{Evapo-transpiration, option 4}}\]