Definition 1 is the most frequently used, but it is entirely inaccurate. Usually, when saint is used in this way it means that a person is very virtuous, admirable, giving, etc. But it says nothing about real holiness. Definition 2 is used frequently within the Church to denote a person who is extraordinarily pious or religious. But usually this usage is meant as a statement of exaggerated praise, not as a theological truth.
We do not necessarily know if our pious grandma is in heaven or not; the use of the term saint in this way is an exaggerated way of saying she was very pious. With definitions 3 and 4 we come to the precise, theological usage of the word saint. Generally speaking, a Saint is anybody who is in heaven, including the angels (who are referred to as Saints in the Church’s liturgy).