Musar zu Wajikra 19:24

ובשנה הרביעית יהיה כל פריו קדש . The reason for this commandment is that during the first three years impurity was an integral part of the tree, causing all manner of כשופים. In the fourth year this spirit of impurity departs. The words ערל, ערלה are expressions of impurity. Once sanctity is introduced such objects become purified. The word הלולים denotes a festivity. It signals that the destructive power which had up to then affected this tree has departed and all of its fruit has from now on become fit to receive blessing.