The best time to do a 4D ultrasound is between 26 and 32 weeks.
Depending on gestation and position of baby you will be able to see facial features, shape of mouth, fingers and toes. You will also be able to see them moving in real time, drinking, yawning and smiling. It really is a glimpse into the secret life of the womb.
You should try and come earlier between 22-24 weeks gestation, if you are expecting twins.