Refeeding syndrome is a serious condition that can occur when a malnourished patient begins refeeding, either enterally or parenterally. It is marked by various metabolic disturbances.

Let's assess each of the options given in the question:

1. Hypophosphatemia: This is the hallmark of refeeding syndrome. When refeeding begins, insulin secretion increases, driving phosphate into the cells and causing serum phosphate levels to drop.
2. Hypocalcemia: While not a classic feature, some electrolyte imbalances in refeeding syndrome can include changes in calcium levels. However, it is less commonly highlighted compared to phosphate, magnesium, and potassium.
3. Hypomagnesemia: Similar to phosphate, magnesium levels can drop significantly. This is a characteristic feature because of cellular uptake driven by increased insulin secretion.
4. Hyponatremia: Although electrolyte imbalances occur, sodium is generally not a key electrolyte that's affected in the classic definition of refeeding syndrome.

Based on the above analysis, Hyponatremia is the exception and is not typically associated with refeeding syndrome compared to the other features listed.

Therefore, the correct answer is Hyponatremia.