Hi, there! Welcome to the rotational gameplay club! I've been playing rotationally since 2015, so I'd be glad to give you some tips.
My first and biggest suggestion would be to have some sort of structure or schedule to when you rotate houses. This can be as rigid or loose as you want, but it will make some of the other quirks about TS4 easier to play around.
Aging is actually the easiest thing to control. Basically, you have 3 options in the gameplay settings: Aging Off (i.e. nobody ages, ever), Aging On (i.e. everybody ages, played or not) and Only Active Household (i.e. only the Sims you're currently playing will age). Option #3 is most likely going to be the one to choose; however, you also may want to switch the game to option #2 for a short period of time every so often, to let the townies age up as well. For example, I personally play each household for 7 Sim days in a set order, so every time I start a new rotation I leave aging on for everybody while I play through the first house. This way, the same child townie won't be friends with 3 consecutive generations of Sims.
Now here's where things get a bit tricky. While it is possible to pause aging when you switch houses, it is not possible to pause time. This is different than TS2; in TS2, when you switched houses your first house was left frozen exactly where you left off. In TS4, though, time will keep progressing in the save file. The following changes will continue to occur no matter which household you are playing:
1) Season progression, If you have Seasons installed, they will continue to change. It is entirely possible to leave a household in summer and return in winter.
2) Sick Pets This is definitely more in the "mild annoyance" category, but if you have pets on your lot, expect them to be sick when you return to the house. I'm not sure if this is a bug or a feature, honestly. I've actually started just purchasing a wellness treat out of the Debug items list so I don't have to start every lot with a trip to the vet.
3) Relationship changes. If your inactive Sims appear on community lots or are invited to an event by your active household, any relationship gain or loss that happens there will be reflected when you get back to playing them. However, it's also important to know that Sims will not get married, break up, or otherwise enter or leave committed relationships without your direct involvement. They just might be slightly better friends, or like each other a little bit less due to that argument at the party.
4) Skill Gain: The same way Sims' relationships will change while inactive, their skills will as well. If your inactive Sim appears and plays on a chess table while your active Sim is at the park, the inactive Sim will gain Logic skill in the process. (There's actually ways you can use this to your advantage; I personally make great use of the playdate system from Toddler Stuff to help all my toddlers max their skills.)
5)Gig Based Careers: For the most part, Sims who are in rabbit hole (Business, Entertainer, Criminal, etc) , active (Doctor, Detective, Scientist) , or work from home (Critic, Social Media, Gardening, etc.) careers will all continue to attend work when you are not playing them. They won't get promoted, but they will do enough to not get fired while you're away. However, the same cannot be said for gig-based careers like Acting or Home Decorator. If you schedule a gig for a Thursday, and switch houses on Wednesday, your Sim will miss the gig. It is still entirely possible to play these careers with a rotational save; you just have to be careful about when you're scheduling gigs. This is one reason why having a plan or structure to when you rotate is important; if you know when you're going to switch houses ahead of time, it helps you to better plan when you should or should not schedule gigs for your Sim.
6) Pregnancy This is a big one. If you rotate away from a house with a pregnant Sim, that pregnancy will still progress while your away, and the baby will be born without you there. The game will also randomly assign the baby a name, which may or may not be ok with you. (Personally, it was very much not ok with me because I theme all my Sims' names.) Fortunately, this is also pretty easy to manage with some planning. Just like with gigs, you want to be sure to time your Sims' pregnancies so the baby will be born before you rotate houses.
7)University This is the most annoying one. Basically, University in TS4 works like a combination of a rabbit hole and a work from home career. Your Sim will have rabbit hole classes to attend, but they will also have specific tasks like term papers and presentations like a work from home career. Your Sim obviously cannot do those when you're not playing them. Therefore, if you switch away from a Sim while they are in University, they will either have failed out or be on academic probation when you return. The only real way around this is to play through the entire term in one go, and don't enroll for another one until you come back. It took me two rotations to find a way to handle University in my game, and it's convoluted as all get out. I would highly encourage you to plan your rotational schedule around University if you intend to use it.
And that's it. Honestly, if you take the time to understand how the game works, most of these quirks are pretty easy to work around. Have fun with your new rotational save!
