My wife has a constant pain, on the right arm pit (ganglion), before her period. What can we do? She has a cyst of 4mm in the right breast.

Polymastia. The axilla is the most common site of polymastia (accessory breast tissue), a benign condition present in about one percent of the general population. The most common symptoms are pain and swelling during periods of female hormonal changes (menstruation and pregnancy). Surgery is not necessary, but may be indicated if the symptoms are severe or for cosmetic reasons.
See a breast surgeon. Your wife should undergo full workup of the cyst and she may consider removing it surgically to help with the pain and to confirm that it is in fact ganglion cyst.