I've enjoyed the Lost Fleet series, and I continued to enjoy it through book 4, despite its flaws. That said, the book fails the basic sequel test: if this had been the first book in the series, would I still be reading the series? At this point, my answer is "probably not".
What's still good?
The military battles are still well-described. There are better writers of speculative military fiction (Charles Stross, John Scalzi) in term of what can generally be described as "thinking up cool, futuristic stuff". Campbell excels in the telling of battles in enjoyable tactical detail in a plausible, futuristic setting. His ability to factor in time distortions, relativistic changes, simple momentum, leadership, motivation, and even navigation was what originally drew me to the series. This talent is still on display in the battle scenes of this book.
What's not so good?
It would be a stretch to say "everything else", but there are some flaws that appear to be worsening over the course of the series.
CAPT John Geary, the fleet's commander, is still the only character who seems fully fleshed out. We spend the books inside his head, and by book 4 much of the Geary internal monologue about honor, duty, ancestors, etc., is a bit repetitive. Still, Geary remains a likable, honorably motivated leader without becoming a caricature. For the rest, not so much.
Victoria Rione, is, to judge by reader comments, almost universally annoying. What's more, while her motivations initially seemed congruent with her actions, that no longer seems the case. She vacillates between insightful advisor and shrewish harridan, and I actually LIKED her character initially. Now I find myself in the camp saying "Please, someone slap her".
CAPT Desjani, the loyal subordinate and captain of the fleet flagship, still seems too 2-dimensional to function as Geary's love interest. Way too many "Rione spoke while Desjani gritted her teeth" sequences. The book has too much of this interplay. I'd bet Geary wishes Campbell would write a holo-deck into book 5 to get him out of this.
I could go on, but you get the idea.
So, if you really have enjoyed the battle sequences, as I have, then the book will probably be worth it. If you struggled through the 3rd book thinking "please don't have so much cat-fighting in the 4th", well, consider yourself warned. If you are new to the series, I can unreservedly recommend the first book, Dauntless.
I don't want to come off as too harsh. I enjoyed this book and plan to buy the 5th. I think fans of the series will generally still enjoy this entry. Still, I do think it's fair to point out what I see as areas to improve in the concluding books.