She and her friends join a new gym that promises instant weight loss. However, the gym is a front for Jadeite , who is using "Shape Rays" to drain energy from the girls.
After being teased by her family and friends for gaining a little weight, Usagi becomes obsessed with getting thin.
In the original 90s anime, Episode 4 is titled (English dub: " Slim City " ). It primarily focuses on Usagi’s struggle with body image and a deceptive plot by the Dark Kingdom. Episode 4 Summary (90s Anime)
At the end of the episode, Usagi realizes that healthy habits are better than "quick fixes," though in the original Japanese version, she is dismayed to find she hasn't actually lost any weight. Episode 4 Summary (Sailor Moon Crystal)
Luna grows suspicious and follows Usagi to the gym. After realizing the trap, Usagi transforms into Sailor Moon to stop Jadeite’s plan. Interestingly, this is one of the few episodes where Jadeite’s scheme doesn't involve a monster (Youma).
If you are watching the more recent reboot, Sailor Moon Crystal , Act 4 is titled .