I'm not the official expert, but in simple layman's terms, here's what I see:
In #1, the background has a lot of reddish coloration that blends well with the coat of the deer, so the deer doesn't pop out. In #2, you don't have that problem, since the background is not that color. In #3, there's a tendency toward the problem, but not serious at that point. Although #4 has the same colorations as #3, because of the way the deer is positioned, it also doesn't stand out as well as in #3.
I've seen posts here describing how to alter backgrounds with layer processing that could help with this.