HI Jack, not sure I understand the description of damage correctly. 3-4 inches in diameter is quite large so would have to be some kind of mechanical damage. peaches weep sap when the bark is damaged. if not that large than could be borers which are comon on peach trees. borer contorl requires spraying the trunk and lower limbs with carbaryl (Sevin) on June 15, July 15 and August 15. please read and follow all label directions.