Unthrifty loveliness, why dost thou spend
Upon thy self thy beauty's legacy?
Nature's bequest gives nothing, but doth lend
And being frank she lends to those are free:
Then, beauteous niggard, why dost thou abuse
The bounteous largess given thee to give?
Profitless usurer, why dost thou use
So great a sum of sums, yet canst not live?
For having traffic with thy self alone
Thou of thy self thy sweet self dost deceive:
Then how when nature calls thee to be gone
What acceptable audit canst thou leave?
Thy unused beauty must be tombed with thee
Which, used, lives th' executor to be
An alternative point for the  volta  may be here, so that the first three quatrains may neatly end with a pause or  caesura . The remaining couplet forms a resolution. 
 The sonnet concludes with the idea that If the young man doesn’t procreate, his beauty will be buried along with him. The alternative is to live on through his children, which his “executor” — the one who will deal with the gains and losses of his life — will note. 
 Shakespeare is playing on the theme of usury and use, characteristic of his deft wordplay.  Here there is a  paradox : instead of being used up, wealth increases by being used.
Sonnet 4