Acaldera is a 1. type of volcano 2. type of volcanic eruption 3. very large volcanic bomb 4. large depression in a volcano

Option 4, Large depression in a volcano

Explanation:

Caldera is a form of volcanic depression or a basin-shaped crater at volcano top which is formed by a large volcanic eruption and collapse of rock into magma chamber. These two events leave behind a deep underground chamber depression. For example - Yellowstone National Park in USA and Valles Grande in New Mexico are example of Caldera. Sometimes these Calderas have active geothermal system.  

Hence, option 4 is correct