To find out how long it takes to screen-print a batch of 4 shirts, we can substitute \( s = 4 \) into the equation \( t = 4s + 3 \).
Substituting:
\[ t = 4(4) + 3 \]
Calculating:
\[ t = 16 + 3 = 19 \]
Therefore, it takes 19 minutes to screen-print a batch of 4 shirts.