What are Macbeth's intentions in his soliloquy during Act IV, Scene 1, and how does the language and structure convey his urgency?

Macbeth's final fourteen or so lines in this scene actually constitute an aside, not a soliloquy, because he is not alone on stage.  Lennox is still with him, but Macbeth speaks in an aside, to himself, and so -- by convention -- Lennox cannot hear him.

Macbeth is upset Macduff has escaped him because Macbeth waited too long to act on his suspicions of Macduff; Macduff is now out of his reach in England, meeting with...

Macbeth's final fourteen or so lines in this scene actually constitute an aside, not a soliloquy, because he is not alone on stage.  Lennox is still with him, but Macbeth speaks in an aside, to himself, and so -- by convention -- Lennox cannot hear him.

Macbeth is upset Macduff has escaped him because Macbeth waited too long to act on his suspicions of Macduff; Macduff is now out of his reach in England, meeting with Duncan's son, Malcolm.  So now, Macbeth intends to act as soon as he conceives of a plan.  He says that the "firstlings of [his] heart shall be / The firstlings of [his] hand" from now on (3.1.168-169).  Therefore, when it occurs to Macbeth to kill Lady Macduff and all of Macduff's children and servants as a way to punish the thane, he vows to do it right away, as if to inaugurate his new resolution.  

When Macbeth lays out his plan for Macduff's innocent and unknowing family, he lets the audience in on it.  Now, we know more than the Macduffs do, and this creates dramatic irony (which occurs when the audience knows more than one or more characters).  Dramatic irony heightens the tension for us, and so this helps to convey the tension and urgency Macbeth feels as well.