In this passage the Nurse is making a pun, or play on words, with the word "teen" (I.iii.16). When asked Juliet's age, Nurse says that she will bet fourteen of teeth that Juliet is not yet fourteen, which we can see in the lines, "I'll lay fourteen of my teeth-- .... / She is not fourteen" (15, 16). The word "lay" can be translated as "bet." However, Nurse uses a parenthetical element set off between dashes to make a pun out of the word "teen" to relate it to Juliet's age.
The word teen can also be translated to mean sorrow. Therefore, Nurse is using "teen" to refer to both her sorrow at only having four teeth and to Juliet's age as a teenager. Hence, we can translate Nurses second line, "And yet, to my teen be it spoken, I have but four--," to mean,"And yet, to my sorrow I confess that I only have four teeth--" (16).
Nurse is also using the number of her teeth, which is only four, plus the play on the word teen, to refer to Juliet's age, which is nearly fourteen. So, if we were to translate these lines fully, we would get:
I'll bet fourteen of my teeth--
And yet, to my sorrow(teen) I confess that I only have four teeth--
That Juliet is not yet fourteen.