To find the total number of people at the stadium, we can use the information that 4% of the attendees were teenagers.
Let \( x \) be the total number of people at the stadium. According to the problem, 4% of \( x \) equals 390 teenagers:
\[ 0.04x = 390 \]
To find \( x \), we can solve the equation by dividing both sides by 0.04:
\[ x = \frac{390}{0.04} \]
Calculating that gives:
\[ x = 9750 \]
Thus, the total number of people at the stadium was 9750.