Prosperity ends at the death of the generation 5 heir. I will have to double check that.
Midnight sun is a legacy on a stupid tiny island. I have one in Sims 2, 3 and 4. LOL You can also pair it with the perfect genetics challenge to make it super hard.
The only rules I found for Sims 4 made it sound like a lot of cheating to make it work. But I am thinking the rules are probably more flexible than the ones I found. I think the one I found said to download a specific lot, cheat your sims into the foundation of the lot so they would be trapped on their island and could never leave. Kids have to never attend school or they would get stuck in the foundation and glitch. (Guess they all fail school over the course of the challenge?) It made it sound like it didn't transition very well to Sims 4, so I guess there is a better rule-set that I couldn't find that you are using? The rules linked above are for sims 3 and sound interesting. I just haven't had time to track down a good set of rules for sims 4.
I am thinking if we need to, we can just make the Sims 3 version fit Sims 4. Instead of an island, buy a lot, surround an x amount of area with water and try that way. Sims 3 was a 10x10 lot. I figure you can buy any size lot, but surround all but 10x10 with water.
Regarding the Prosperity Challenge, I know in the past we held off on that one because no one's ever been close to finishing one (as far as I know/we knew then). The same was true with the BaCC, until Twilightoutside finished one, at which time we added a medal.