Note: Much of this information can be found in the article on Rotational Play that's on the Guide.
Click here for the guide page.
As of November, 2017, a new Culling system has been put into effect. More information can be found
here. Basically, any household you play should be put into "My Households" to avoid being culled. This replaces any previous information about culling.
However, I'll be adding new info in posts to this thread, so scroll down to see what's new. Keep an eye on dates -- as things are added (and taken away) in patches, the information in an earlier post might be superseded by a later.
One of the things I enjoyed in TS2 was playing several families in the same file. That wasn't easy to do in TS3, so I was happy to see that I could return to rotational play in TS4.
Initially, there were some problems in the game that made rotational play difficult, primarily losing played sims during a cull. I've found that if sims are in a household, they do stay within the game if you don't play them for a few sim weeks.
Another problem reported is that married sims get involved in romantic relationships with people they're not married to. I haven't had this to any great extent, but I don't have many sims with romantic traits or aspirations that would make romances more likely. I should say that Bella Goth, who does have the romantic trait, has not gotten involved with anyone other than Mortimer. My sims have occasionally made friends (and enemies) autonomously, though.
Babies will be born after the 3rd day of pregnancy whether you are playing the mother or not; pregnancy also lasts 3 days even if you have aging off. The game supplies a bassinet automatically, but it also provides the baby with a name. I usually play a pregnant sim until the baby is born, but if I don't, I go back and age up the infant, so I don't know if babies become children automatically. You can send anyone into a house to age up a baby as well; you won't get to choose the aspiration and trait then. One odd feature I've had -- a couple of mothers who have given birth when I haven't been playing them have had twins.
I've occasionally had a child complete an aspiration while I was not playing him or her, and I suppose the same thing could happen with adults, especially if they were left with everything done except leveling up in a skill. The game won't automatically choose a new aspiration but will keep them on the old one. They won't make progress on their jobs, but I think children and teens might manage to become B students.
You can play with aging off and age up your sims manually. If you're playing with 4 households, the long setting will give you the same ages as the normal setting if you're playing your households for the same amount of time. Using the "age active household only" with the normal setting works also.
Tip #1 -- Keep hard copy records. I use index cards, but notebooks work too. At the end of each week I've played a sim, I put a checkmark next to their age category. I also keep track of their traits, aspirations, and jobs.
Tip #2 -- Don't check the "Fill Empty Houses" box.
I like to keep some cheap houses open for adult offspring to move to and for sims like the Goths and Spencer-Kim-Lewises to move into when they can't afford their current homes anymore.
Tip #3 -- Use "Repair Game" as needed and each time you change families, if you're playing each family for a week in each round. It's a good idea to clear your cache file, too.
I tend to regard small glitches as a sign that my file is getting too large. When Summer Holiday walked through 5 walls and a couch to get from the kitchen to the living room, I decided it was time to start a new file. It took me about an hour to move my 11 households into a new version of Willow Creek.
1. Go to "Manage World"
2. For each household that you intend to move, choose "Edit household."
3. Once in Edit household, choose "Save to My Library."
4. When you have saved all your households, go to the houses that you have built or significantly altered. Click on the "build mode" symbol and choose "Save to my Library."
5. Save and exit to menu.
6. Choose new game.
7. You'll start in CAS, so go to the gallery, go to your library, and pick one of your households to replace the sim in CAS. Move this family to an empty lot.
8. Go to Manage World and click on Edit household for each of the filled houses. Once in Edit household, choose Delete. If you just evict the family, you'll have a second version running around town in addition to your version.
9. Bulldoze the houses that you have altered and replace them with the ones from your Library.
10. Go around your neighborhood and make a note of the prices of each of the houses that you intend to move your sims into. I've found that the "freerealestate" cheat doesn't work. Your families will come into the game with only 20,000 simoleons (or a little more, depending on family size).
11. Go back to your family on the empty lot and click "Play". Do Ctrl-Shift-C, and type "testingcheats true" in the white space. I use the "motherlode" and "kaching" cheats to give them enough money to buy back the house they originally lived in, but you can give them an exact amount, too, with "Money X".
12. Go to Manage Worlds again. Click on the "Move out" option for your family, and move them back into their house.
13. Click on the empty lot, and choose "Move in." When you're in CAS, pick go to Gallery. Go to your library and pick another family to replace the sim in CAS. Then click on the check mark at the bottom.
14. Keep doing steps 11, 12, and 13 until you've resettled everyone.
15. Once you've finished moving in all your families, go into Manage Worlds, then Household Management (upper righthand corner). Check your "played households" and make sure everyone is there. Then go into "unplayed households." If you have extra copies of a family, delete them.
In your new file, your sims will have the same traits, aspiration progress, job progress and household relationships that they had before. They will, however, lose their relationships with sims outside the household, and these sims will not show up in the family tree. I don't know what happens with whims -- they might still have a whim they had before but it will be unpinned.
I do enjoy playing rotationally, and I'm looking forward to GTW, when I can have my families buy stuff from the shops owned by other families. It's fun seeing them make friends with each other, and occasionally they surprise me, like when Summer Holiday and Ronan Chappell started a romance when I wasn't controlling either of them.
I have a whole story going (the link is in my signature), but I thought I'd provide a more basic "how to" post as well.
Edited to add step 15 -- I discovered a second Summer Holiday running around town.