I've read the previous 3 books in this series as well. Let me say they are very entertaining and hard to put down. But the problem with them is two-fold:
1. No character development
The books move way too fast, you are left wishing they would slow down a little bit and Larson would more fully develop the background story as to the other aliens, the mysteries of the universe he setup, the military strategy, the political issues, etc. That being said, this is a minor problem compared to the love story.
2. The girlfriend damn near makes the books unreadable
Without the underlying love story between Kyle Riggs and Sandra, the books are already stretching (but not quite reaching) semi-ridiculous. Thanks to the action and plot weaves and the uniqueness of the universe that Larson creates, he makes up for these surface-level deficits in a big way.
That being said, Sandra nearly ruins the books completely. In the first place, she is way too involved in every aspect of the story. No one would buy a military commander bringing his girlfriend everywhere with him in the middle of a war. Even worse, the unprofessionalism of it all is just snickered and waved off by all of the characters around them, even the inner circle (even though Larson makes them out to be strict no non-sense pros in every other way). Worse than that, Riggs even comments that he shouldn't keep bringing her everywhere, but that is followed by another adventure in which she is literally on his tail and inserting herself into everything he does with his approval.
Though I could give many examples from the first four books, reading chapter 22 of this particular book was the absolute height of folly. I don't want to give away too much of what happens, but lets just say that when computer aliens attack your planet and your headquarters, sending hundreds of ground troops to surround you (after just destroying most of your headquarters), the general on-scene wouldn't decide to go off to the half-destroyed mess hall for a gourmet dinner and get drunk all night. Then, with enemy ships still visible in the sky, proceed to go on a 36 hour vacation to have sex with his girlfriend off on the beach in some far corner of the island (with enemy ships visible in the sky overhead, no less). This is an example of where this series goes from truly entertaining sci-fi (if not B level) into a cartoon.