Signs do not rule houses. Planets do.
If the 4th cusp is found in the sign of Capricorn, Saturn rules 4th House affairs.
If Mercury is in the 4th he is ruled by Saturn.
Suppose you have Libra on the 4th cusp.
Venus rules the 4th.
But suppose Jupiter is in in the 4th but in Scorpio.
Then Mars is his ruler (or Pluto if that is the system of rulership you use.)
But because Venus rules the 4th, "Affairs dealing with home, family, land, etc.", then it is also necessary to examine Venus' role in Jupiter's activities.
Venus, her condition and aspects, sets the tone for 4th House matters.
Mars directly disposes Jupiter.
It may be more informative if for the word ruler, or lord, we substitute "dispositor". A planetary ruler "disposes" the planets in his sign (and houses where that sign occurs) to behave in a certain way, in accord with the condition and aspects of the dispositor. The dispositor is the "lord" of the house...read the dictionary definition of "dispose" and "disposition". This should help you to understand the way that a ruler (lord, dispositor) influences the planets under its dominion (aha, another cool word to look up in the dicionary.) The lord of a house is its "suzerain". How about "sovereign"?
As an example,
I have two planets in the 4th House...
Neptune is in the last degree of Virgo (disposed by Mercury), very near the cusp of the 4th (or IC).
Libra is intercepted in the 4th and contains Moon. Venus is her lord, but again, Mercury is primary ruler of 4th House matters.
So without hesitation, we can say that Mercury and Venus are "co-rulers" of the 4th.
Interesting to note that the arc-distance between Neptune ("dissolution") and Moon is 27 degrees...If we allow that one degree equals one year, then it takes Neptune 27 years to direct to Moon. At the age of 27, my marriage, home, family was dissolved. The two co-rulers of the 4th are both in the 8th (not good), retrograde (not good) and combust (very not good)...so the direction of Neptune to Moon gave the results that it did. Were Mercury and Venus in good cosmic state with happy aspects and so on, the outcome of the direction would have taken on a different face.
But their unfortunate position (the 8th is a house of misfortune) disposed 4th House affairs to take an unfortunate turn. Naturally, other directions and transits brought their effects with them at the time of the dissolution.
In summary, planets rule...signs do not. Planets are active and dynamic, thus capable of rulership. Signs are fields of energy that directly affect whatever they may contain, but do not act outside of their own boundaries. But of course if a powerful planet lies within a certain sign, and is therefore modified or affected by the nature of that sign (and its lord), then the sign does have effects outside its own limits...but only through the planet it contains. Loosely, we might consider the signs as "areas of dominion" of their lord; planets within a sign are thus subject to the moods and policies of the lord of the sign.
Planets within a house act directly and immediately on the affairs of the house, according to their inherent and accidental nature. These planets are "tenants", or we can see them as the subjects of the lord. Rulers, if in a different sign and house, act "at distance". But they nonetheless "dispose" the planets under their rulership. Planets within a house are more apt to be prominent during earlier years, but the "final outcome" of the house will tend to follow the condition of the lord later on.
ex: If you have Saturn in the 11th, you would say: 4th house ruler in the 11th house. But yes, this is very confusing to explain.
