The grammatical subject of the sentence is "Americans." The grammatical subject of a sentence is a single noun or pronoun (unless there are more than one subject linked by a conjunction like "and"). Modifiers do not form part of the subject. It's hard to see the grammatical subject here because it is modified by a complex comparative, "four times as many...as"
In simpler form, the sentence would be
More Americans were killed in the Battle of Sharpsburg, MD....than [the number of Americans who] would later be killed on the beaches of Normandy...
Instead of "more (Americans)...than..." the writer uses "four times as many (Americans) ...as." The expression "four times as many (Americans) as..." is the same kind of modifier as "more (Americans) ...than...."