The monument for the Fourth United States Infantry is on the north side of the Boonsboro Pike east of Rodman Avenue. (Boonsboro Pike tour map)

From the marker:

U.S.A.
Fifth Army Corps

Fourth United States Infantry (8 Cos.)

Captain Hiram Dryer, 4th U.S. Infantry, Commanding.

(September 16-17, 1862.)

On the morning of the 16th, the 4th Infantry seized the Middle Bridge over the Antietam: Companies B, G, I and K were thrown across the bridge, deployed on either side of the road, advanced about 300 yards, and, between 2 and 4 P.M., became engaged with the enemy’s skirmishers. They were relieved at sunset by the I st Battalion, 12th U.S. Infantry and recrossed the Antietam. Between 2 and 3 P.M. of the 17th, the Regiment again crossed the Antietam, under orders to support the Horse Batteries. It advanced beyond the Batteries and took position in the field to the right, the left resting on the road at this point. Companies G, I and K, under Lieut. C. H. Carlton, were deployed as skirmishers and, advancing in a westerly direction, encountered the Confederate skirmishers posted behind a rail fence. The skirmishers suffered severely from an enfilading fire of canister from Cemetery Hill, south of the turnpike, and from the enemy’s skirmishers in their front.

At about sunset the skirmishers were withdrawn and the regiment, carrying its dead and wounded, recrossed the Antietam.

No. 85.

See more about the history of the 4th United States Infantry Regiment in the Civil War