I'm going to get that our of the way, cuz I know someone will say this: Sims 4 is an alternate world and isn't part of Sims 4 timeline.
Personally, I hate this alternate world thing and I act as if Sims 4 were after Sims 1 and before Sims 2, jsut like you did, cuz Bella still didn't disappear, Mortimer's still young, Cassie's still a teen. BUT, some thing would be all over the place, like the Pleasant sister/Dustin and Dirk being older in Sims 4 than in Sims 2, so I act as if each Sims 4 world were in a different timeline.
Sims 3: Mortimer is a kid here(Kinda all over the place, some worlds are in the present, some in the past and some in the future. I've calculated where each of them would be placed in the timeline using sims ages/presence in world as a parameter, but I'll share it later. I'll set in as the past cuz most worlds are set in the past).
Hypothetical Oasis Springs 1: Ignoring the presence of too older Dina/Nina/Don, the Landgraabs could exist here. Johnny Zest is a good sim, just like Sims 3 Malcolm Landgraab used to be, while Sims 4 Malcolm is evil. I think Johnny is the original Malcolm, but Nancy disinherited him as he didn't want to take up on the family business to become a comedian. Later she had another kid which she name after the original Malcolm Landgraab. Johnny changed his name to Johnny, and adopted his father's maiden name Zest.
Sims 1: Mortimer and Bella are young adults with a child Cassandra.
Sims 4 Willow Creek + all the other worlds that don't have any classic premade sims: Mortimer/Bella are older young adults now, Cassie is a teen and they had Alexander now.
Hypothetical Oasis Springs 2: Ignoring the Landgraabs being younger than they should be. Dina and Nina still live with her mother and Don, and they still didn't move to Pleasantview and Dina still needs to marry with Michael Bachelor, a famous elderly football player.
Sims 4 Gallery 1: The Pancakes had a son named Iggy.
Sims 2: Bella disappeared, Mortimer is older, Cassie is an adult now and Alex is about to become a teen.
Sims 4 Gallery 2: Loki Beaker cloned himself and had two kids named Atom and Ceres.
Sims 4 Gallery 3: Malcolm Landgraab, son of Nancy, married Hannelore and had Adrien Landgraab. Later he divorced and married Mom Landgraab and had Dudley and Mimi Landgraab.
Sims 4 Britechester: Dustin Broke probably didn't had money to go to university. The Pleasant twins turned into YA and moved to uni. They probably broke up with their boyfriends in the meantime, or not.
Sims 4 Del Sol Valley: After finishing uni, Dirk Dreamer becomes a celebrity. Dustin Broke also turns into a celebrity.