The marker for the center of the line of the Fourth United States Infantry is on the east side of Richardson Avenue north of the Boonsboro Pike. (Boonsboro Pike tour map)

From the marker:

Fourth United States Infantry

The Fourth United States Infantry, Captain Hiram Dryer commanding, crossing the Antietam between 2 and 3 P.M., of September 17, 1862, advanced to with in a few feet of this point. Three Companies under command of Lieut. C.H. Carlton were deployed as skirmishers and moving on the high ground in the direction of Sharpsburg became sharply engaged with those of the enemy. The skirmishers were withdrawn about sunset, and shortly after, the regiment retired across the Antietam. This tablet marks the center of the advanced line.

No. 86.