The fourth chakra moving upwards is Anahata Chakra, which resides at the heart and is associated physically with the heart and thymus gland. Anahata in Sanskrit means something like "unhurt." Whereas the first three chakras deal with the self's relationship to itself and the body, the fourth chakra is all about relating to others. Just like the second chakra, Anahata is a realm of emotion, but in this case it's about emotions relating to others-love instead of desire, how others see us instead of how we see ourselves. Like the third chakra, it is associated with making decisions, literally "following your heart."
When the heart chakra is under-active, we can feel closed-off from others, defensive, and anxious. If it is over-active, we may rely too much on what others think about us, and may have problems with the lungs or skin. A well functioning, balanced Anahata Chakra enables us to feel connected to the world around us, compassionate, and full of love.
The element associated with Anahata is air, so lots of pranayama, or breathing exercises, will stimulate this chakra. Giving to charity, performing acts of mercy, and acting with compassion will also bring balance to the fourth chakra.
thanks again to the comprehensive work of Anodea Judith for much of this information