Two layers of rulership
The first layer is the sign. Aries has been assigned to Mars since the Hellenistic period, on the scheme that arranges the planets outward from the luminaries, and no later revision moved it. Mars carries the meanings of heat, assertion and quick decision.
The second layer is the decan. Each sign divides into three bands of ten degrees, and the Chaldean order gives them planets running from the slowest to the fastest, starting from the first degree of Aries. That places Saturn on the opening of the zodiac in one common variant and Mars in another, and by the second band of Aries the sequences in ordinary use have settled on the Sun.
Rulership in this material is a scheme of assignment rather than a physical claim. It says which planet a tradition treats as speaking for a stretch of the zodiac, and reading it as anything stronger than that overstates what the texts themselves set out to do.
Fourteen degrees, and where that sits
At noon across 1930 to 2035 the Sun on 4 April has stood between roughly thirteen and three quarters and fifteen and a quarter degrees of Aries. That is comfortably inside the ten to twenty band, far enough from both edges that no year in the sample lands in a neighbouring decan.
The window is narrow because the leap year cycle holds the calendar against the orbit. Three years of quarter degree drift forward are cancelled by one leap day, and the position oscillates instead of wandering.
So the useful answer to what degree am I is a narrow window plus your year, not a single figure copied from a table. Tables of dates cannot resolve below the level of the sign.
How the tradition combines them
A Mars sign with a solar decan is read as the assertive quality of Aries directed toward being visible and toward holding ground rather than simply seizing it. Older texts sometimes describe this stretch as the part of Aries most concerned with standing rather than starting.
That is inherited interpretation and should be marked as such. Nothing in the astronomy says a solar band means anything at all. What can be checked is the position; what is being handed down is a convention for describing it, and different schools describe the same ten degrees differently.
What no rulership scheme can tell you
Neither layer says anything about the rest of the chart. Mars itself is a moving body, and on 4 April it may be anywhere in the zodiac depending on the year, so an Aries Sun does not imply an Aries Mars.
The Moon changes sign roughly every two and a half days and often mid way through this date, and the Ascendant turns through the full zodiac every twenty four hours. The widget above works out the solar degree for a specific year, and returns the Moon, the Ascendant and the distance from the nearest sign boundary from a birth time and place.
A chart is a set of positions, and the date supplies one of them accurately. The rest arrive with the hour and the place.
Questions people ask about this
Which planet rules a 4 April birthday?
Mars rules Aries, so that is the sign ruler in every scheme. The ten degree band this date falls in is assigned to the Sun, which the tradition treats as a qualification rather than a replacement.
Does my Mars have to be in Aries too?
No. Mars orbits independently and takes about two years to circle the zodiac, so it can be in any sign on this date depending on the year. Only the Sun is fixed by the calendar.
What degree is the Sun on 4 April?
Near fourteen degrees of Aries, within a window of about a degree and a half across the last century. Your own year decides where in that window you sit.
Is the modern ruler of Aries different?
No. Aries kept Mars when the outer planets were assigned to signs, unlike Scorpio, Aquarius and Pisces, which each picked up a modern ruler alongside the classical one.
Why do some sources give a different decan ruler?
Because there is more than one scheme. The Chaldean order runs the planets by apparent speed, while the triplicity scheme starts each sign's bands from the ruler of its element. For this band both routes arrive at the Sun.
Does a decan ruler affect a horoscope reading?
In practice most popular readings ignore it entirely. Where it is used, it narrows the description of a sun sign rather than changing which sign the Sun is in.