The correct answer is option 4. It is difficult to measure the coefficient of expansion of a liquid compared to a solid because liquids are contained in a container, and when heated, both the liquid and the container expand. This makes it challenging to accurately measure the expansion of the liquid alone.

Option 1, which states that liquids tend to evaporate at all temperatures, is not the main reason why it is difficult to measure the coefficient of expansion. While evaporation can affect the measurement of the liquid`s volume, it is not the primary factor that makes measuring the coefficient of expansion difficult.

Option 2 suggests that liquids conduct more heat, but this does not directly relate to the measurement of the coefficient of expansion. Heat conduction is relevant for understanding how energy transfers within a substance, but it does not affect the measurement of expansion.

Option 3 states that liquids expand too much when heated, but this is not specific to liquids. Both solids and liquids expand when heated, but the difficulty in measuring the coefficient of expansion of a liquid is mainly due to the expansion of the container.

In summary, option 4 is the correct answer because the expansion of the container when heated makes it challenging to accurately measure the coefficient of expansion of a liquid.