Why the hell would you want it between MGS2 and 4?
It'd just mean any instance of it being weird or outrageous would be completely out of place, and all actions ultimately so unimportant that they're left completely unmentioned in 4, which means either a ton of boring and unsurprising filler or just plain bad writing. Not to mention you have to default back to Raiden as seen in MGS4 by the end, and work from Raiden as seen in MGS2. That's a stiflingly bad range to have to cover.
Taking place after 4 means anything could happen. No clue what's going to occur after this. I'm way more interested now.
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