In Act 4, scene 1, Shakespeare makes fruitful use of meter. The Weird Sisters speak in trochaic tetrameter, often in rhyming couplets. This means that the standard line has eight syllables divided into four feet (tetra-), and the first syllable of each foot is accented while the second is not. For example, in the following, I will mark stressed syllables with bold font and use the "|" mark to indicate breaks between feet:
Dou ble | dou ble | toil and | trou ble
Fi ire | burn and | caul dron | bub ble
Use of this trochaic meter, as well as the rhyming couplet form (where every two lines share an end rhyme), makes the sisters sound not only otherworldly, but menacing. The heavy rhythm begins with a stressed syllable rather than leading up to the accent, making them sound rather threatening and aggressive. Further, the rhyming couplet structure lends itself to their incantations; it sounds like they are spell-casting (which they are!), and it's a bit hypnotic.
Further, many of the spell-casting lines are actually truncated, meaning that the final (eighth) syllable of the line is missing. For example,
Round a | bout the | caul dron | go
In the | poi soned | en trails | throw
Notice that there are only seven syllables per line, and the final unstressed syllable is missing in these lines. This missing syllable leaves us with the feeling that the there is more to come, that the witches have not finished manipulating Macbeth; it's almost like a verbal equivalent of foreshadowing. We are waiting for that last syllable, just as we await to see what happens to Macbeth next.