the term circumstances is used in moral theology in a way that is different from its secular dictionary definition. It is anything that pertains to the morality of the act, other than the first font (intention) and the second font (the act itself and its inherent meaning). Principally, this would refer to the consequences of the act.
1. intention: to save others from death
2. act itself: an act of commission, implementing the positive precept to love your neighbor, preventing their deaths at the cost of his own life, but without violating the negative precept against suicide. His act of throwing himself on the grenade had the inherent meaning of directly protecting others from injury and death, and indirectly resulting in his own death.
3. good consequences, four persons were saved from death or severe injury, outweigh bad consequences, one person dies.
If the situation were that he would have to put a gun to his own head and kill himself directly in order to save a large number of persons, it would not be moral. One cannot directly do evil so that good may result.