Learn German: Du misst genau vier Milliliter. - You measure exactly four milliliters.

In German, masculine and neuter nouns used as units of measurement usually don't change in the plural. Since der Milliliter is masculine, it stays Milliliter even when talking about four of them.

Where does the verb misst come from?

It comes from the verb messen (to measure). It is a strong verb that changes its stem vowel from "e" to "i" in the second and third person singular: ich messe, du misst, er/sie/es misst. Also, because the stem already ends in an "s" sound, the du form just adds a -t instead of -st.

Can I put genau at the very end of the sentence?

It is much better where it is. Placing genau right before vier Milliliter emphasizes that it's exactly four milliliters that you are measuring. If you put it at the end, it sounds more like you are describing how you measure (doing the action in a careful, precise way) rather than the exact quantity itself.