There are seven major energy centers or psychic "organs" which provides and distributes energy throughout your subtle/non-physical or "auric" body. These are called chakras.
The third eye or brow chakra is the sixth major chakra in your subtle body.
Its located at the middle of your forehead though according to some tradition its attatched to your pineal gland ( the idea is that our ancestors had an actual third eye which later evolved into our pineal gland ) .
Be that as it may, its job is to provide you with higher understanding, connect you with your higher self, help you observe aura or aid you in OBE's, clairvoyance e.t.c .
As for your last question, I don't think humans had a fourth eye, but I might be wrong.
The chakras are associated with the endocrine system, it is not that the pineal glad was an actual third eye, it is just part of the endocrine system. There is no material evidence to suggest humans had a physical third eye at all.