What indicates relative strength of an electric field?

 As with gravitational field lines, each field line has a direction marked on it with an arrow that shows the direction of the electric field at all points along the field line. The relative strength of the field is indicated by the density of the field lines (that is, by how close the lines are). The more lines there are in a given area, the larger the field.

It's because 3 if it was 10 it wouldn't look right do it goes up to 4d