I have never used any mods in TS4. I bought Seasons a couple weeks ago or so, and have been testing it out on a savegame I had only played for a couple weeks back in May 2016. Today I experienced a winter massacre.
Couple odd things I noticed before that happened: a large number of NPCs who'd been human the last time I'd played the game were suddenly all showing up as undisguised aliens. I've never seen so many aliens in any of my other savegames, and I don't know why they suddenly all showed up undisguised. I'm not investigating all my other savegames until the massacres are fixed, so I don't know if the alien-issue is an issue in all of 'em or not.
Second odd thing is that my doors no longer lock. I get the lock options, and I lock the doors, but every sim in the world can waltz through my played-sims' house in spite of the locks. The vampires feed on my main sim about every other night.
As to the massacre: I'm playing a merry-widow/gold-digger sim. I sent her newlywed husband to the gym in Willow Creek, quickly put in a pool in the back, and sent him skinnydipping so he could freeze to death. He eventually exited against my wishes, clothed himself against my wishes, and went out back to mourn Don Lothario, who had turned into a popsicle without my noticing. Large crowds of sims gathered around, mourners froze to death (including my sim), you know the drill. I brought another of my household sims to the lot (y'know, click on their pic & bring them there), put him in his winter clothes, and tried to rescue the in-world npcs. Weird stuff started happening when I tried to stop the die-off.
Geoffrey Landgraab was in his jogging clothes, and blue-skinned. I repeatedly commanded my sim to ask Geoff to show off his winter outfit, but the game wiped my command out of existence & sent my sim through the back gate, put him into his swimsuit, and sat him on the edge of the pool about to go in. All this was contrary to my own inputs for that sim. I managed, each time, to get him to put his winter clothes back on, but the only way I could keep him IN the clothes was to teleport him away from the pool. I'd then try to get Geoff, or Dina Caliente, or some of the other sims into their winter clothing so they wouldn't freeze, and the game would take control of my sim away from me, every time, and send him back to strip by the pool.
I did manage to save most (but not all) the npcs by pleading with Grim and giving him death flowers (my sim's a master gardener & well-supplied), but asking the other sims to show me their winter outfits usually (but not always) sent my sim to the edge of the pool.
My guess is that interfering with the mourning &/or witnessing deaths &/or witnessing resurrections was part of the problem.
Either that, or EA programmers are evil. >:)
My sim, btw, is a master athlete & has the long-lived trait, & that's probably part of the reason he kept trying to go swimming. Maybe? He's not that insistent on exercising under ordinary circumstances.
It was pretty wild, though, keeping him from committing swimmicide. (Yes, yes, pun intended. Simmicide + swim.) ;)
ETA: @AprilDawn: " Geoffrey Landgrabb, Nancy Landgrabb,... Darling Walsh, Don Lothario, Brant Hecking, Bjorn Bjergsen"--these were all present today behind the gym, & most of 'em froze. My sim was able to rescue Geoffrey & Brant from Grim; Nancy was in her winter clothes, Darling did not turn blue, and the last I remember of Bjorn was his frozen popsicle. I simply can't remember if my sim managed to rescue him from Grim or not. He also got Eliza Pancakes resurrected. A personal relationship with Grim and a lot of death flowers are quite a help!