Can you reduce areola from 6 cm to 4 cm?

it can be done , but the scars would not be great.  too much pleating.  if it is part of a mastopexy is more likely to be pleasing

Your current areolar circumference is 28 cms, and based on your desired diameter of 4 cm, the new circumference would be 12.6 cms. In other words, the surgeon would have to sew one skin edge that is over double the length of the other skin edge, and this would not happen without the usual problems: severe pleating, a thickened scar, re-expansion of the areola, and flattening of the breast mound.  

About 2 centimeters of skin can be removed circumferentially in a circumareolar mastopexy. At 2cm the inner and outer circle can be taken in. Even with this by definition there is pleating. So 9 cm to 5 cm is reasonable. The pleating will usually flatten over time.  The nature of this lift is a radial tension that can lead to scar widening and areolar stretching so the risk must be worth it to you. The more tissue removed the more tension, and pleating. Sensation is usually maintained in the skin that is left behind. Some breast ducts do drain through the areola so they may be altered with this surgery, but ducts to the nipple and remaining areola will suffice in most cases.Best wishes.

An exam is needed to know for sure, but it is possible in general to reduce the areola size to 45mm. An areola reduction of any degree will still have risks associated with it. I encourage you to visit with a board certified plastic surgeon near you to discuss your concerns and learn more about your options.