Shinku Chitai : The Isolated Zone vol.4 (Japanese) is available on Amazon now.
Then, because a new book has been released, I'll have a free book campaign as usual. In this time, The Isolated Zone (Japanese) vol.1 and vol.2 (not vol.3) If you are waiting for vol.3's free campaign - sorry, it will be when vol.5 is published. It's from 26th to 30th.
At the moment, The Isolated Zone Japanese e-book is KDP version only, but the series will be released from some Japanese publisher. After that, I can't keep this low price (100JPY, about 1$) and I can't have any free campaign, so if you are interested in, hurry up. ;)
About the English edition, it's stopped for a while, but I had got the dialogue translation vol.2 and 3 recently and sent back the doc.files with my comment. There are still many things to be done before the release, but it's started finally, at least. ^_^