In PS1, Motovia was the first planet from the star, but in PS4 it is the second one.
In PS1, Motavia was hot and arid, because it was near the star.
In PS2, the planets had artificial systems controlled by Mother Brain to control the environment, including the ecosystem and the climate.
If Mother Brain was created by the Earthmen, how did the people of Algol start using her? If she were a "gift", they would know who created her.
In PS4, these systems continue existing to keep the life without Mother Brain, but both planets could live without them in PS1. Furthermore, Motavia is in a better orbit.
It is not clear who created Zelan and Wren. Wren was created after the destruction of Mother Brain.
In PS4, the people from all planets returned to the Middle Ages. In PS3, they returned to the Middle Ages too.
How did the high technology that existed in PS1 disappear from the surface?
My best guess is, Palma was the center of both technology and industry for all three planets--the other two were essentially wastelands in PS1 anyway, so it backs this up. The loss of Mother Brain left an empty seat of power and nobody with the technology or might to fill it.
Another strange thing - How can a satellite destroy a planet?
If it is so easy to destroy planets with a satellite, Lassic could have used satellites to destroy planets when he was the ruler of Algol in order to free the Profound Darkness.
What is the relation between the Earthlings and the Dark Force? The Earthlings appeared many years before the release of a new Dark Force and wanted to occupy Algol because Earth had been destroyed.
The Dark Force destroyed Palma to weaken the seal of the Profound Darkness. However, the Earthlings said that they destroyed Palma and that is strange, because if the Earthlings wanted a new planet to dwell in, then the destruction of the best planet of the system would be the last thing that they would do.
Why did the Palmans want to arrest Rolf? He cannot be blamed for Mother Brain's malfunctions, because he started his mission when the problems were already occurring and Neifirst is the real culprit.
If Phantasy Star V were developed, I wonder who would be the enemy in Algol since the Profound Darkness is dead. Maybe the game would be set in another solar system.