Volume 4 continues the Dark Moon clan arc that inspired the second part of the Sailor Moon R series. The alien tree part was an addition to kill time for this to be finished.
One part I loved was in the first chapter a sick Mokoto in a drugged up state opened up to Motoki about her past and even a bit about the their powers. Motoki, and later Naru, aren't your usual oblivious bystander. They realize that something is going on with their friends and they want to help, though they may not be able to. I hope to see this expanded on in future volumes.
We also get to see Mamoru consult his stones which carry the souls of his past generals for guidance. They encourage him to support Usagi as her future king. One nice change from the anime, is Tuxedo Mask does actually develop new powers so he can continue to fight alongside Sailor Moon and help her. As opposed to his rose throwing ability.
We also get to meet Sailor Pluto, the solitary guardian of the Gate of Time. She does allow the guardians to pass, but not without trying to kill them first. I would like to see what she can do when she doesn't hold back and hopefully we can someday.
Another good volume. It's a much more condensed story that the anime, but I enjoy it more. I hope it gets adapted into the new anime. The Dark Moon clan is one of my favorite villains, and I did like their overall goal.