The text
says: “Power was given to them over a fourth of the earth, to kill with the sword, with famine, with death, and by the wild beasts.”
Analysis
The fourth seal introduces Death and the realm of the dead. The text specifies that their power extends over a quarter of the earth. The means of death mentioned (sword, famine, pestilence, wild beasts) overlap with the judgments already introduced by the previous horsemen (war and famine).
Interpretation
Not a cumulative addition: the quarter of the earth is not the sum of the deaths of the previous seals plus those of the fourth.
The previous horsemen (red and black) announce war and famine as realities.
The fourth seal encompasses these realities and associates them with death itself.
An overall figure: the quarter of the earth represents the total portion affected by Death and Hades, including the effects of wars, famines and pestilences.
In other words, the deaths from war and famine are counted within this quarter, since they are explicitly mentioned as instruments of Death.
Symbolic scope: the quarter is not an exact statistic, but a limit set by God.
This shows that the judgment is severe but not total. God keeps sovereignty by setting bounds to the extent of death.
Synthesis
The deaths linked to war and famine are included in the quarter of humanity struck by the fourth seal. This figure is not added to the previous ones, but represents an overall whole: Death and Hades are given power over a quarter of the earth, through all the instruments of destruction (sword, famine, pestilence, wild beasts).