When does overdrive kick in?

i have a 1991 S-10 with the automatic and if i drive it in the D without the circle around it i have only three gears but if i drive it in the D with the circle around it i have four gears and the fourth one kicks in about 40mph i was wondering if that was overdrive and maybe i only have a 3 speed automatic with overdrive but i wasn't sure if that was an option in 91 on the dash there is the P R N D(with circle) D 2 1 does that mean i only have a 3 speed with overdrive?