“And in the fourth year all of its fruit shall be holy to give praise to Hashem!” (Vayikra 19:23-24)
It is a positive commandment that all of the fruit of the fourth year should be completely holy. This means to say that all of the fruit produced by a tree in the fourth year after its planting will be holy, and must therefore be eaten by their owners in Yerusholayim (like Maaser Sheni). This constitutes its holiness, as it says “And in the fourth year all of its fruit shall be holy to give praise to Hashem!”.
Amongst the roots of the commandment are that Hashem wanted people to be aroused to praise Him at the beginning of the best of the fruit of his trees, in order that the pleasantness and blessing of Hashem will rest upon him and his fruits will be blessed, for Hashem wishes only good to His creations. He therefore commanded us to bring them up to the place that he chose from the earliest days as the place to serve Him and to eat them there, for it is there that Hashem commanded that there will be blessing. The best of the fruit of a tree are those which grow in the fourth year. There is also another purpose in His commanding us to eat some of our fruit in Yerusholayim (e.g. the fruit of the fourth year and maaser sheni and maaser behemah), and that is that this will cause each person to fix his own or some of his sons’ place of abode in that place in order to learn the Torah, for it is yjere that the masters of the Torah dwell and there lies the source of wisdom.
This commandment applies to men and to women only in Eretz Yisrael (and not outside) and only when the Beis Hamikdash was standing. There are those who say that it applies even nowadays and outside Eretz Yisrael. It therefore follows that anyone who owns fruit from the fourth year of a tree, must redeem it using money.