This is a hard book to rate because the author, in every other book in this series, fails to actually tell a story with a beginning, middle and END. Here, he does do that but he does so by having his main character strong-armed into going to Asgard. So, in essence, this is a side story because all the main story lines about how the MC, Daniel Black (DB), is preparing to survive Ragnarok with as large a group as possible are quickly forgotten about. Some headway in surviving Ragnarok is made in that the DB gains a lot of information and couple more allies that will contribute to survival. However, this is really the story of DB being mistaken by Odin for an Atlantean wizard and taken to Asgard to fix an Atlantean magic item (and how DB survives in and escapes from Asgard).
So the DB's mission at the end of book 3 appears to have succeeded in stopping, for the time being, Gaea's attacks on Kozalin. As a result the DB is able to further prepare for Ragnarok. This includes fixing up the dock area adjacent to his castle and taking a 20,000 year old dryad as his familiar.
Then the DB is asked to visit the local Norse pantheon priests. Knowing that the priests have been decimated in recent attacks DB is certain they don't have enough power to hurt him so, even though the requested visit is suspicious, DB goes. To DB's great surprise Odin and Thor are waiting for him and take him to Asgard. It turns out that centuries before Atlantean wizards (AW) attempted to overthrow the gods. The AW built a magical device capable of imprisoning gods for centuries but were defeated. Some of the AW escaped destruction by providing for their own reincarnation. Since DB and AW both have virtually limitless mana supplies, Odin concludes that DB is a reincarnated AW. This is important because Odin has the AW device for imprisoning gods and needs it repaired.
Once DB is taken to Asgard he knows he is in trouble. DB recalls how the Asgardians sabotaged the building of their own walls to avoid paying the giant who built them. So DB doesn't believe Odin will reward him and set him free once the device is repaired. Most likely Odin will either kill DB to keep the device secret or make DB continue working for him. Further, Asgard provides all the evidence necessary for DB to conclude that Odin is exactly the type of person that should not have the repaired magic device.
Worse, word that DB is an AW has leaked. The Egyptian god Ra was so pissed at the AW the he established a group of assassins to hunt them down. These assassins, with divine support and several thousands of years of experience, are extremely powerful, including being able to infiltrate Asgard. So DB has to dodge very powerful assassins while attempting to simultaneously keep Odin happy and not repair the device.
Then DB encounters Aphrodite and Loki's daughter, Mara, and things get really difficult. The Greek goddess of love has been a sex slave in Asgard ever since the Olympia was destroyed by Asgard. Aphrodite is aware that she has in part brought her fate upon herself by being completely untrustworthy, and yet that is her nature so it remains an open question if she can reform sufficiently to aid in her own rescue. Mara has infiltrated Asgard on Loki's business and brings DB news of Ragnarok.
Bottom line: This is the first book in this series I wouldn't have minded buying, but it is also a side story in many respects and so doesn't indicated that book five will be any better than books 1-3.