4th Pennsylvania Cavalry

"In the movement into Maryland, the army being again under General McClellan, the Fourth had the advance until it reached Frederick City, when it was assigned to General [sic] Averill's Brigade. The illness of the latter preventing him from taking the field, the command of the brigade devolved upon Colonel Childs, that of the regiment upon Lieutenant Colonel Kerr."

[Although Bates made this statement about the brigade command in Maryland, Colonel Rush of the 6th Pennsylvania Cavalry actually commanded the brigade in Maryland per the ORs and Carman.]

"The brigade crossed the Antietam with the troops upon the left, and was posted in front of the Stone Bridge, where the Fourth supported Clark's Battery and held the line upon its right. A single solid shot, which fell in the midst of the squadron supporting these guns, killed two men and four horses. Colonel Childs was among the killed in this battle."
(from Bates)

Capt Young led a squadron of the Regiment in advance of the Regular Infantry (as skirmishers) in the afternoon of 17 September in their advance toward the town of Sharpsburg along the Boonsboro Pike.

Detail Map #10: Confederate Counterattacks in the Center (11 am-1 pm)
Detail Map #11: Sykes' Regulars Advance from the Middle Bridge (noon-4 pm)

Tablet #122: Army of the Potomac - 17 Sep, 10 AM to 17 Sep, 6 PM
Tablet #91: Third Brigade, Cavalry Division - 17 Sep, 6 AM to 17 Sep, 3 PM
Tablet #21: Cavalry Division - 17 Sep, 6 AM to 17 Sep, 8 PM
Tablet #121: Army of the Potomac - 17 Sep, 8 AM to 17 Sep, 6 PM

Source: Bates, Samuel P., History of the Pennsylvania Volunteers, 1861-65, Harrisburg: 1868-1871, extracted online at Pennsylvania in the Civil War. See more about the Regiment and its field officers on behind AotW.