The fourth house is a pivot, angular, and in the Hellenistic scheme it is the Hypogeion, the subterranean place. Counting by whole signs it stands square to the rising sign, so it is configured to it.
Its cusp is where a very common misunderstanding lives. In the quadrant systems, Porphyry, Alcabitius, Campanus, Regiomontanus, Placidus and Koch, the fourth cusp is the lower meridian, the Imum Coeli, exactly opposite the Midheaven. In the Equal house system it is not: there the fourth cusp is ninety degrees along the ecliptic from the ascendant, and the lower meridian falls wherever it falls, which at high latitude can be a long way off. In whole-sign houses the fourth house is simply the fourth sign from the ascendant, and again the lower meridian can sit in the third, the fourth or the fifth.
So the statement that the fourth house is the Imum Coeli is true of one family of systems and false of the other two, and the same chart can put a planet at the very bottom of the sky in either the third house or the fourth depending on which is used.
Because the fourth is the house of the home, the land and the ancestral line, and because those readings are often the most personally weighted a client hears, the system used deserves to be stated before the reading is given rather than after it is questioned.