Let the number of pants be \( 4x \) and the number of shorts be \( 3x \). According to the problem, the total number of pants and shorts combined is 70. Therefore, we can establish the equation:
\[ 4x + 3x = 70 \]
Simplifying this, we get:
\[ 7x = 70 \]
Now, divide both sides by 7:
\[ x = 10 \]
Now that we have \( x \), we can find the number of shorts:
\[ \text{Number of shorts} = 3x = 3 \times 10 = 30 \]
Thus, the number of shorts available for sale is \( \boxed{30} \).