The Fourth Astrological House - Subterraneous Place
The Fourth House: Also known as the subterraneous place.  Because this is the place closest to the earth and is where things most naturally come to rest, it symbolizes burial and the state of the natives affairs just after death and the legacy we leave behind. Along with this, it symbolizes the season of old age, of ancestry and blood relations (particularly parents), and where we come from. It represents the foundation of our security, the derivation of our identity and our sense of where we belong. Because it is the most hidden of the places, it symbolizes why many find it difficult to completely know themselves and that which remains secret. In western society this was considered the place of patrilineage, but with the increasing association between the houses and the signs in modern times (the 4th house being like Cancer) this became the house of the mother, and by derivative houses, the 7th from the 4th (the 10th) became the house of the father. There was a similar controversy caused by a mistranslation of a preposition in Ptolemy that led to the Mother being assigned to the 10th house in ancient times, now having been cleared up by Rob Schmidt. While the controversy isn't settled, I now tend to assign both parents to the 4th house.
On a material level it is the ground upon which you stand and represents the home, foundations, estates and non-movable property. In some of the translations this place is also said to promote the aquisition of ships. If the lot of Fortune is here and you own land, barring indications to the contrary, the property may be thought to contain hidden wealth, or treasure. The ancients also considered this place to be a contributing cause of the ability to have children, along with the 5th, 10th and 11th places. In ancient times, children were considered one's retirement in old age (the 4th). Modern society now has Social Security to take it's place so the contributing places seem to take a back seat to the 5th as representing children today.
More info on each of the planets that have dealings with this place: