*Each* is a [Quantifier][1], a part of speech that wasn't discovered until the 19th century,  too late to get into the [Top Eight list][2], which was canonized much earlier. Quantifiers are a form of Determiner (another POS), and they "bind" noun phrases, which means they modify and quantify them. Like most noun modifiers, quantifiers are naturally found before the noun they modify.

Like many quantifiers, however, *each* is subject to a syntactic rule called "Quantifier Shift" (aka "Quantifier-Float" or "Q-Float"), which moves a quantifier from a prenominal position to an adverbial position:

 - ***Each** of the boys said they would go.* == Q-Float ==> *The boys **each** said they would go.*

Q-Float applies to the quantifier *all* as well as *each*, but **not** to the quantifier *every*:

 - ***All** the boys said they would go*. == Q-Float ==> *The boys **all** said they would go.*  
 - *Every boy said he would go*. == Q-Float ==> _*The boy(s) every said he/they would go._

As to the question...

In the first sentence in the question, *that each have 4 strokes* is a relative clause modifying *letters*, with _that_ as the subject relative marker, and a Q-Floated *each*. 

In the second sentence, *each has 4 strokes* is half of a compound sentence, a main clause with *each* as its subject. 

*Each* (and, again, *all*, but not *every*) can act as a pronoun substituting for (in this case) *each letter*, in the same way _this_ can substitute for _this letter_.

  [1]: http://en.wikipedia.org/wiki/Quantification
  [2]: http://www.thelatinlibrary.com/don.html