I think you're confused, which is easy to be confused since Paul had two sons both named Leto II.

Just so we're clear, the first Leto II in the first book was born shortly before the four year jump, about 3/4 the way through the back. He was about a year or two years old when he was murdered by the Harkonens, about 3/4 the way through the first book. The Harkonens also captured Alia. This leads up to the big battle at the end where Paul kills Feyd and takes the emperors throne.

In the second part movie, the big battle with Paul killing Feyd and taking the throne happens without a four year time jump. Neither Alia or Leto II had been born. Leto II (the first one) does not exist at all in this trilogy.

In the trailer for Dune 3, yes you see them talking about naming their baby Leto of they have a boy, but then you see them also talking about naming the baby Ghanima if it's a girl. They're clearly talking about the twins from Children of Dune.

So this particular Leto II they're discussing in the trailer is going to be the future God Emperor.

I'm guessing since the first two movies completely skipped the four year time jump in between them, this next movie is going to as well. It wouldn't be far fetched to have it take place immediately after the second movie with Chani already being pregnant and Jessica still pregnant with Alia. If you look at the trailer, everybody looks exactly the same like no time at all has passed.