In Sonnet 4, Shakespeare uses money or banking imagery to symbolize wasting one's youth on selfish pursuits instead of marrying and having children. The speaker admonishes the listener to "spend" all of his or her time on selfish pleasures. He speaks of "sums" and "largess" and calls the listener a "usurer," or a person who charges excessive interest on loans.
Nature gives us our youth and beauty freely, but they won't last forever. The word "niggard" is an archaic way of saying "selfish" or "miserly"; so we shouldn't be selfish with what we have. We should marry and have children while we are young. In the last three lines, the money imagery returns with "audit." When we are gone, what will come out in our audit? Will we seal up our beauty in the tomb?
This poem is urging a young man to marry and produce offspring. The reason the speaker gives to the man is that because the man has been given such a gift of beauty by nature, he owes it to nature to pass it on.
Shakespeare uses the metaphor of a financial gift or inheritance to symbolize the "gift" of beauty that has been given by nature. This symbol is first established in the second line with the word "legacy." The speaker asks the man why he only "spends" this "legacy" upon himself - in other words, why doesn't the man share his beauty? In the next line, Shakespeare refers to the beauty as "Nature's bequest", and in line 6, the beauty is "bounteous largesse."
After using the metaphor to reinforce that the beauty has been a gift for nature, the speaker then turns the symbol upon his speaker, asking what "audit" the speaker can give in return for the gift? The only answer, of course, is for the speaker to have children and continue the gift he has been given.