Donne's metaphysical conceit in "A Valediction:  Forbidding Mourning" only comes into play in the final three of the nine stanzas.  Donne uses a total of four comparisons in the poem, one of which is the conceit that involves the legs of a compass.

Here are the four comparisons, stated directly for you:

1. Separation of death compared with separation when one lover leaves another (stanzas one & two).  Let we two lovers not cry or sigh, but keep our separation to ourselves.  The idea is that to speak loosely about their feelings is to lose them.
2. Movement of the earth draws attention to itself, yet movement among the stars, which is movement of far more importance, goes unnoticed (stanzas three-five).  Their love is like the movement of the stars.  It doesn't need to draw attention to itself to be monumental.  They don't need to cry or make a show of their separation.
3. Their love does not suffer a breach, or break, but experiences an expansion:  like gold that is beaten to airy thinness (stanza six).
4. Their love is like two legs of a compass.  One leg travels around, but is always connected to and anchored by the other.  Two legs of a compass cannot be fully separated, just as the two lovers can never really be separated. 

A fifth comparison actually exists as well.  I combined two for the sake of simplicity and because you asked for four, but stanzas four and five are really a bit of a variation on the movement of the stars idea.  In these stanzas, a certain area of space cannot allow for any separation among its parts because its parts are what make it what it is.  But the lovers, in stanza five, being refined and knowing each other's thoughts so well, can handle separation of their parts, such as eyes, lips, and hands.

John Donne's "A Valediction:  Forbidding Mourning Group" contains a metaphysical conceit, an extended metaphor or simile in which the poet draws an ingenious comparison between two very unlike things [enotes].  In this poem, the comparison is made with Donne and his wife, who must part temporarily and two compasses.

The main idea of Donne's poem is that in their parting (he sojourned in France), they should not profane their love.  While the love of "sublunary lovers" cannot survive separation, theirs easily can.

For, the love that he shares with his wife is "interassured of the mind" and it surpasses the limitations of such mundane circumstances as their temporary parting. 

In this parting, they experience, not an end, but an expansion likened to the "gold to aery thinness beat." 

If their souls are separated, Donne contends in his metaphysical conceit that they are like the feet of a compass:  "as stiff twin compasses are two."  His foot is the part of the compass that moves around the fixed foot, that of his wife: 

Thy firmness makes my circle just

And makes me end where I begun.

Donne's love transcends the mere physical in this conceit of the simile of his and his wife's two feet as the points of a compass.