«Unthrifty loveliness, why dost thou spend
Upon thyself thy beauty’s legacy?».
The themes of narcissism and usury (meant here as a form of use) are most developed in this sonnet, with its references to wills and testaments. The terms “unthrifty,” “legacy,” “bequest,” and “free” (which in line 4 means to be generous), imply that nature’s generosity should be matched by those who benefit from it.
Unthrifty loveliness, why dost thou spend
The poet, who calls the youth a “beauteous niggard,” or a miser of his good looks, claims that his young friend abuses the many gifts of beauty nature has given him and thus is a “profitless usurer,” a business term that recalls the three previous sonnets.
Sonnet 4 summarizes all that the poet has been saying thus far. In a series of questions and statements, the poet lectures about the wise use of nature, which liberally lends its gifts to those who are equally generous in perpetuating nature by having children. But the youth’s hoarding contrasts to nature’s bountifulness. Lines 7 and 8 express this contrast in terms of usury: “Profitless usurer, why dost thou use / So great a sum of sums, yet canst not live?” The term use here means both invest and use up. Similarly, “live” means both to gain immortality and to make a living.
The inevitable conclusion is that if the youth does not properly use his beauty, he will die childless and doom himself to oblivion, but if he fathers a child, he will be remembered. The final couplet presents these contrasting possibilities. Line 13 uses familiar death imagery to express the negative result of dying childless: “Thy unused beauty must be tombed with thee.” However, line 14 suggests that should the young man use his beauty to have a child, an “executor to be,” his beauty will be enhanced because he will have used it as nature intended.
English audio from YouTube Channel Socratica
Summary from Cliffsnotes.com