Season 4 - Headmasters and Targetmasters. There is heavy focus on Nebulos. Pretenders are introduced towards the end of the season. The series has a bit of an identity crisis because it doesn't know if it wants to take place on Earth, Cybertron, or Nebulos. The conflict between Galvatron and Scoroponk continues. Galvatron is phased out by the end of this season.
Season 5 - Powermaster Optimus Prime. More Headmasters, Targetmasters, and Pretenders. More focus on Earth since Nebulos is secure, and to narrow down the setting after the confusion of the previous season. Micromasters introduced.
Season 6 - With the exception of Optimus, most of the Headmasters, Targetmasters, and Powermasters are phased out. Micromasters and Pretenders mainly on Earth and Cyberton.
Season 7 - Megatron returns, somehow the Transformers lose the ability to Transform and become Actionmasters. Pretenders and Micromasters are still around but it's like Nebulos and the other ____masters never existed.