The part of the brain that is also known as the "little brain" is the cerebellum. The cerebellum is located at the back of the brain, underneath the cerebrum. It is responsible for coordinating voluntary movements, balance, posture, and motor learning.

Option 1: The cerebrum is the largest part of the brain and is responsible for higher brain functions such as thinking, memory, and perception. It is not known as the "little brain".

Option 3: The thalamus is a structure in the brain that acts as a relay station for sensory information. It is not known as the "little brain".

Option 4: The hypothalamus is a small structure in the brain that plays a crucial role in regulating various bodily functions such as hunger, thirst, body temperature, and hormonal balance. It is not known as the "little brain".

Therefore, the correct answer is option 2 - cerebellum.