Hi! It's possible that it's a softlock - someone else reported a bug with retrieving party members in current release.
What exact location is telling you that need all four party members?
I appreciate the bug report!
I need the four party members for the diving to the lake on the elf route. I checked the save editor and apparently my party members are [1,2,3,4], even though I am alone and I could do the Mayor route as well as the solo side quests. I could, though, go to the clearing in the forest and set up camp with Jen appearing there. Oh, and they're not in real life either. I can't find the three of them there.
Coincidentally, the solo side quest requires me to go to the lake as well, so uh I can't do that either.
(I took on the quest after I realized I softlocked everything)