The monument to Battery E is south of Gettysburg on Bushman Hill, south of South Confederate Avenue and west of Big Round Top. (39.78468° N, 77.2473° W; Google map; Tour map: Bushman Hill & Slyder Farm) The battery is also referenced on a War Department marker at Antietam.
From the monument:
Army of the Potomac
First Brigade Horse Artillery
Battery E, Fourth U.S. Artillery
Four 3 inch rifles
Lieut. Samuel S. Elder commanding
July 3 Arrived on the field and took position on a hill southwest of Round Top and engaged under Brig. General E. J. Farnsworth in the afternoon against the Confederate right.
Casualties: killed 1 man
Signs (below) mark both ends of the steep trail that loops off South Confederate Avenue to the hilltop monument.