The correct answer is option 1: "As ice melts, it expands uniformly up to 4°C."

This statement is incorrect because as ice melts, it actually contracts, or becomes smaller in volume. This phenomenon is due to the unique structure of water molecules in solid and liquid form. Water molecules form a lattice structure in the solid state, and as it melts and transitions to the liquid state, the molecules become less tightly packed, causing the volume to decrease.

Option 2 is correct. A mercury thermometer works by using the principle of expansion due to heating. As the temperature increases, the mercury inside the thermometer expands, causing it to rise in the narrow tube, indicating the temperature.

Option 3 is also correct. Keeping a small gap between two rails allows for expansion due to heating. When materials are heated, they tend to expand, and if there is no space for expansion, it can lead to deformation or structural damage.

Option 4 is also correct. When the temperature of a metallic wire increases, its length increases as well. This is because heat causes the metal atoms to vibrate more rapidly, leading to a larger average distance between the atoms and hence an increase in length.