Since \( xx \) and \( yy \) vary directly, we can express this relationship mathematically as:
\[ yy = k \cdot xx \]
where \( k \) is a constant.
Given that \( yy = 15 \) when \( xx = 3 \), we can first find the value of \( k \):
\[ 15 = k \cdot 3 \]
To find \( k \), we divide both sides by 3:
\[ k = \frac{15}{3} = 5 \]
Now we have the direct variation equation:
\[ yy = 5 \cdot xx \]
Next, we want to find \( yy \) when \( xx = 4 \):
\[ yy = 5 \cdot 4 = 20 \]
Therefore, \( yy \) when \( xx \) is 4 is \( \boxed{20} \).