4 months post op, my hair color is difference in beard transplant?

By the description you gave, I am assuming you have had scalp hairs put into your beard area. In most people, scalp hair and beard hair have slightly different color and thickness, so it is common to have a mismatch. This, however, is usually not that noticeable. As the hairs mature a little more in their new location, it is possible that the difference in color and texture will improve. If not, you can always color the hairs in order to make the color more even.

Beard hair transplant with scalp hair can look different.  This is because scalp hair can look different than beard hair.  This is an unavoidable fact associated with beard hair transplant.  If your beard hair is light and scalp hair is dark or grey, it will not change over time.  You can consider coloring your beard hair.

The color of the transplants will reflect the source of the hair. If the hair was taken from the scalp, then the scalp color will prevail and probably not change with time. In time, the texture of the hair may look more beard like, but only time will tell. 

the color of the hair matches the color of the hair where it was taken from in the donor area.