Even in a sellers market, it’s very common for builders to sell the models 6 months (or more) before they’re finished with the project.
In today’s strong buyers market, builders have another reason to sell the models early: today’s price is very likely to be higher than next year’s price.
I don’t think the builders will stop building in 4S. The development of 4S is 2/3 finished. And since it’s a master-planned community, they’ll finish building everything out.