You are right, but aren't zombies technically not B.O.Ws? They are infected humans and some are (like in CV) reanimated corpses. B.O.Ws are the creatures specifically engineered to be used as weapons like the Lickers, Tyrants, etc. Although the virus itself is obviously a biological weapon.And it was never about any specific monster. It's always been about specific characters doing their best to survive and avert a crisis with a plethora of different BOWs. Zombie's were some of the main BOW, but they were always just a distraction from the bigger BOWs. That's even how it was in RE1-3. That, IMO, is what Resident Evil is all about. That's why I believe RE4 (and RE5, to a point) didn't stray too much from the core RE concept.
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I really really love RE after RE4. In fact RE4 is my favorite game ever, and the favorite RE game. It has the best action, it's very tense and strategic. All the other TPS games are more twitch action, RE5 a bit less but RE6 totally. They require you to be fast and move around. RE4 is all about careful positioning and deliberate motion, you wouldn't just press the aim button for no reason like in say Mass Effect.
It stands to date as the only third person shooter of its kind. While I love RE5 for being fun, and will love RE6 for the same reason based on the demo, I can't help but feel sorry Capcom didn't want to make a true sequel to RE4. It seems the previous experience of getting stale scared them into trying to innovate and evolve too fast. There was time after RE4 for "more of the same", too bad it was unused.
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I really have to also add that Resident Evil 4 had the best story in the series. Far away from the anime-ish Wesker and the faux Americana of the first three games it was just balls to the walls camp. Very knowing, as well. RE5 seems to not know how ridiculous and B movie its story and cutscenes are, but RE4 does. When Leon says "Rain or shine, you're going down" you can just feel him smirking, looking at you in a knowing fashion going "I know, right?". To wit, RE4 has many fun one liners like "Hasta Luego" or "Your right hand comes off?" while RE5 only has one, the extreme makeover line.