People assume that because 1, 2 and 3 came out four years apart, then four years was all Sims 4 had, but there are technical reasons.
When Sims 1 came out, it was built on limited tech. It wasn't even real 3D. Have you ever seen the base-game? There were a ton of Sims games that did just okay, and it wasn't expected to be the next Sim-City. When it proved to be a cash-cow with the potential for expansions, they got started on something bigger and better and actually 3D and most of all, easier to add to. This was Sims 2.
Sims 2 however had limitations, the moment they released it, they knew it'd have limitations. Even as the expansions mounted up they planned on a smoother, more credit-card friendly version with some online play that ultimately ran on the same engine, and graphical improvements. The engine of Sims 2 and 3 is the same, which is why it was easy to make - it's basically "Sims 2: The Reboot". Sims 3's photorealism was always a bit hit-and-miss though, and graphics were making leaps and bounds. They crammed all they could in, but knew straight away that it would have similar problems.
And so the entire point of Sims 4 is that you can add expansions almost indefinitely and never, ever have to stop for lag reasons. Even the cartoonish art style was designed to be long-life and simple to animate, and the focus on young adult life drew in new, younger players when it released.
So it's not a case of 'usually we get a Sims Game' - it's not like the yearly release of FIFA. The games weren't specifically planned to come out four years apart - Sims 2 took three-four years to make, and ran out of room four years later and they knew it would happen, Sims 3 simply had the game engine prepped and ready for when that happened. Sims 4 solved the space limitation problem but made a whole different engine. So Sims 5 didn't need to be ready.
Eventually the base code will become too twisted to add to, which is when we'll get a Sims 5 that will be ready for this sort of thing, and probably some kind of Sims 4.5.