I wouldn't have expected a portal fantasy to wrap up the series, but it is a clever concept to wrap things up, twist the previous tales and do and undo previous activities, and throw in some unexpected curve balls.
I enjoyed it almost as much as book three, with the way it handled old characters, new ones and particularly their involvement in a number of timelines. A couple of things seemed a bit easy for OGD with the pacing, but the story was a good laugh and nice to see The Recorder dragged back down to earth.
If you need a palette cleanser, or quick and funny entertainment it is hard to go past this series. It adds a spin to a number of 'tropes' and the quirkiness works well.
https://www.goodreads.com/book/show/55763789-og-grim-dog