Let the full price of each wool hat be represented by \( p \).
Since the wool hats are selling for $4 less than the full price, the sale price of each hat is given by: \[ p - 4 \]
Jeffrey buys 5 hats at the sale price, so he pays: \[ 5(p - 4) \]
According to the problem, this total cost equals $120: \[ 5(p - 4) = 120 \]
To solve for \( p \), we start by distributing the 5: \[ 5p - 20 = 120 \]
Next, we add 20 to both sides to isolate the term with \( p \): \[ 5p = 140 \]
Now, we divide both sides by 5 to solve for \( p \): \[ p = 28 \]
Therefore, the full price of each wool hat is: \[ \boxed{28} \]