There are no cheats for triplets or quads. But it can happen naturally.
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You need the Insimenator.
There is a cheat but it does not work, so no you cannot have triplets.
Not naturally, if you want to have triplets then you have to have a download a mod for multiple births.
You'll need a mod for quads. Check around Mod the Sims or search google for Nraas. One of those sites will have something for you.
You can only have triplets.