@Mrchipps
Well I am pretty sure that the PvE content should be challanging enough on the current planets. In videos dungeons have been mentioned, sadly we have never seen one, but it was also claimed that without the proper setup you won´t be able to do the dungeon, so they should be challanging.
Considering that all gear is player craftet, the monsters on the 4th planet would then only be good for? what? learning skills?
Getting the rank 3 on a skill already requires you to go to a different planet, which in itself is a hard journey.
If you want to have a hard time collecting resources go visit Taratas and try to collect there.
So I don´t know what a 4th planet like that would add to the gameplay