We knew part 5 was comming, but problem really is how long this series is. It does not make it easy with 3 different volumes going at the same time, but they clearly wanted the manga artist from 1+2 to do vol 5. Given now that vol 2 is finished I'd reckon we see first chapter in 3-6 months.
Publishing aside there were talks about a lot of vol 3+4 apparently had to get a lot of Okay from the author, not sure where or when things went wrong. But it seems very unstable going from 1.5 to 2.5 months per chapter idd.