Diastereomers having more than two chiral carbon and configuration at only one carbon is different are epimers of each other..
Configuration of chiral carbon at D - glucose is 2R,3S,4R, 5R whereas configuration of chiral carbon at D - galactose is 2R,3S,45,5R.
Galactose is C−4 epimer of glucose because configuration at 4th carbon is different.