4. the answer is...

a change (to the Constitution)
an addition (to the Constitution)

An amendment is a change or addition to the Constitution. The Framers of the Constitution knew that laws can change as a country grows. They did not want to make it too easy to modify the Constitution, the supreme law of the land. The Framers did not want the Constitution to lose its meaning. For this reason, the Framers decided that Congress could pass amendments in only two ways: by a two-thirds vote in the U.S. Senate and the House of Representatives or by a special convention. A special convention has to be requested by two-thirds of the states. After an amendment has passed in Congress or by a special convention, the amendment must then be ratified (accepted) by the legislatures of three-fourths of the states. The amendment can also be ratified by a special convention in three-fourths of the states. Not all proposed amendments are ratified. Six times in U.S. history, amendments have passed in Congress but were not approved by enough states to be ratified.