First off the series was not dropped.
Now regarding the schedule, typically at J-Novel the standard practice over here is that once a volume is fully translated there is a 1 - 2 week break before the next volume goes up on the site.
Vol 4 just finished up last week on the 6th but that was a special case more then likely Volume 5 will go up on the 31st of this month or shortly after...
The reason it's not listed in the schedule is cause they can't actually put it up in the schedule till the volume cover is prepared and approved.
Last thing about the schedule I have said this in the past and I stand by it the release schedule at J-Novel is forever changing until a month is fully over the release schedule for that particular month is not set in stone.
Also one last thing about Smartphone specifically normally the volumes were released to subscribers in 6 part batches but going forward that will be in the more tradiational 8 part batches.