There seems to be some confusion on this as it gets asked a lot, here and on the discord. I'm pretty sure it works this way.
The public version is the game, which is version .8.1.8 at this time. If you subscribe to:
Tier2 you will get to start a Great House, acquire a custom slave and start a family quest line along with extra cash and a unique revolver as a reward for subscribing.
Tier3 is the same as Tier2, but you get the public version a week or so earlier before public release.
Tier4 gets everything Tier2 does plus it has some enhanced starts that are basically OP, but saves you having to began at level 1, so to speak. What it gets is access to all the changes and additions that Grim is adding to the game between the public releases. This works as a QC check of the game to find bugs, balance issues, etc. as he works to smooth everything out before the public release. He is constantly updating this and sometimes drops new versions weekly, it seems. You can follow the game progress on his discord.
If you quit at anytime from one of the tiers and go back to the public release you will have all that you have built and earned before opting out. You can downscale and then receive the additions from that Tier only. Certain things in Tier4+ might not be available to the public release so those you would lose, i.e. slave market (the ones you had acquired are there, but no renders for them since those won't be included in game until .8.2 is released.
You will no longer get any more updates on the family quest line or have options of the enhanced starts from Tier2-Tier4+when starting a new game.
If you contributed at Tiers higher than 4 to have a custom quest, custom npc or some custom renders created for the game I would assume those remain as long as they have become part of the public version.
If I erred on any this feel free to correct.