I've been reading the official English version. I got to the end of volume 3, and it seemed like the end, felt like the end, said "Replica The End" at the end (instead of like the other volumes, which say "Replica 1/2 The End"), yet everywhere I look, it says there are 4 volumes to this series. MAL goes so far as to say there are 22 chapters in all, and the last chapter in volume 3 is chapter 15.

Before I pester DMP with questions, can anyone shed light on this? I can imagine the last seven chapters flounder into mediocrity, and this is considered the "good" ending. But I thought I'd throw the question out there...

I have no way of finding out for sure, but I can offer you my theory based on Google's clunky translations. From the comments on volumes 3 and 4 of Replica on Amazon Japan, I get the impression that volume 3 is the end of the main story, and volume 4 seems like it might have been extra chapters released online rather than in a magazine. I checked the publisher's website, and they do seem to have a system for releasing chapters online and via mobile app. As for the actual content of volume 4, the one comment came out too garbled to make much sense, so your guess is as good as mine whether it's a continuation/epilogue/side stories, or what-have-you.

I'm late saying this, but thank you very much for your response! That is interesting - thank you for taking the time to research!

You're quite welcome. Funny coincidence, I finally bought the volumes myself and they showed up literally just days ago. XD