A size 4 soccer ball is a football whose size and weight is suited for players between the ages of 8 and 12 who are actually required to use such a ball size.
Youth players in this age group have outgrown size 3 soccer balls (suited for ages 8 and younger) and aren’t old or big enough to be playing with a size 5 football yet (geared toward ages from 13 up).
The particular dimensions of a size 4 ball help avoid injuries that could occur with a bigger and heavier ball and allow youth footballers to develop and improve their soccer skills.