The distance the particle travels is the area under the v-t graph over the time interval from 0 min to 4 min. The reason you can't just multiply velocity by time is that the velocity is changing at a constant rate over the given time interval (the line is going upward--the particls is speeding up).
What's the area under the graph?
The red outline forms a trapezoid. The area of a trapezoid is where the <em>b</em>'s are the lengths of the two parallel base and <em>h</em> is the height (the distance between the bases).
Reading the graph carefully (notice the scales!)...
