I'm surprised nobody seems to have mentioned this yet, but Ningiyau Tweeted a while ago about the reason for the hiatus. Apparently volume 3 was crowdfunded, and the sponsors were promised specially drawn illustrations in return, but there turned out to be more of them than expected, so she's been spending all her time drawing those instead of getting on with the manga. She's a victim of her own success.
It says there are more details in the image, but I'm too lazy to attempt a translation. Any chance of a summary, mycro?
I thought I posted about that here before, but I must not have. The details in the image are pretty much the same. She didn't expect the crowdfunding bonuses and ensuring their good quality to take up that much of her time, so she needed more time before she could work on the main story again. When it would resume publication, you'd be able to hear about it on her own and School Zone's Twitter, but that's already happened now. I probably should have included this in the Twitter collection if only because of Kei-chan wearing Yokoe's jacket.
I wish I'd started reading this series sooner. Then I may have been able to get in on that, too.