In Shakespearian Sonnet 20, what are the three quatrains and one couplet?

The sonnet is defined by its adherence to strict rules of structure, meter, and rhyme. In Elizabethan sonnets, we see a structure that contains three rhyming quatrains and a couplet, resulting in fourteen lines, written in iambic pentameter. The rhyme scheme runs ABABCDCDEFEFGG. By looking at the rhyme scheme of any given Elizabethan sonnets, you should be able to isolate those various rhyming quatrains and couplets.
Shakespeare's Sonnet 20 adheres to this same structure of three quatrains followed by a couplet. Thus, if we were to isolate the poem's first four lines, it would read as follows:

A woman's face with nature's own hand paintedHast thou the master mistress of my passion,A woman's gentle heart but not acquaintedWith shifting change as is false women's fashion

If you look at rhyme scheme, you'll find that these four lines fit together, with "painted" rhyming with "acquainted" and "passion" with "fashion." This represents the first quatrain.
Next, we come to the second rhyming quatrain, which follows a new sequence of rhyming pairs.

An eye more bright than theirs, less false in rolling:Gilding the object whereupon it gazethA man in hue all hues in his controlling,Which steals men's eyes and women's souls amazeth

Now the rhymes alternate between "rolling/controlling" and "gazeth/amazeth," differentiating this group from the first quatrain. At first, these might seem somewhat difficult to locate—Shakespeare's sonnets are not divided into separate stanzas, so all these various sub-parts are presented together in an unbroken text. But even so, it remains possible to restructure the poem and write each quatrain out as a separate stanza to better differentiate between them. You'd be left with three stanzas consisting of four lines each and then find two remaining lines (the couplet) separate at the end.
In the next four lines, the rhyme scheme seems weaker than in the earlier quatrains, alternating between "created/defeated" and "adoting/nothing," but even so, note that the structure is maintained. Having separated out this third quatrain, you'll find two lines remaining, which read,

But since she pricked thee out for women's pleasureMine be thy love and thy love's use their treasure.

These last two lines comprise the rhyming couplet. Thus, taken as a whole, we have fourteen lines divided into four sub-parts: three quatrains (comprising an ABAB rhyme scheme) followed by a two-line rhyming couplet.