A vehicle travels \( 4 \, \text{km} \) with speed of \( 3 \, \text{km/h} \) and another \( 4 \, \text{km} \) with speed of \( 5 \, \text{km/h} \), then its average speed is :
\( 3.50 \, \text{km/h} \)
\( 4.25 \, \text{km/h} \)
\( 4.00 \, \text{km/h} \)
\( 3.75 \, \text{km/h} \)