Not so much different, but second one is grammartically accurate.

The first line was 저는 미국인 친구가 네 명 있어요. When I wrote my upper reply, there was only 저는 미국인 친구가 네 명 있어요. In the first line. But when I pressed "enter", the result was like that. Oops!

저는 미국인 친구가 네 명 있어요. 저는 미국인 친구 네 명 있어요. also sounds ok. We use it. But I think the first line is perfect.