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:
This opens rather negatively, with the statement that it’s profligate for the Fair Youth to spend his beauty  on himself, instead of investing it by procreating. This is also a veiled reference to masturbation. It echoes the theme of self-love established in  Sonnet 1 , when the poet writes that the Fair Youth is “contracted to thine own bright eyes”. 
 This is the first sonnet to use the imagery of money and wealth in relation to love. Another example is  Sonnet 29  where the Bard refers to loss of “fortune”, but “rich in hope” in respect of his love, and the “wealth” that he wouldn’t exchange for that of kings.  The theme can be tracked throughout this (and many other) sonnet.
Sonnet 4