Lets assume that your argument that he planned fewer story lines is correct. That does not preclude a 4 year arc. The initial concept could easily have included the idea that he would be expanding on it as he went.
If the claims are accurate however wouldn't you expect the first season to be more episodic as the arcs had yet to begin converging? It would show the arcs and stage by stage they would coalesce. New arcs could be inspired and added in by the way in which things were implemented. Sinclairs fiancée could easily have been intended for the role played by Sheridans wife, with the departure of Sinclair the wife was no longer usable for that role so Sheridans was cast for it.
Babylon Squared in the first season set the stage for the 3rd season theft of Babylon 4 and the explanations as to how and why the first 3 Babylon stations were destroyed by the Shadows or their agents.
The Soul Hunter previews the rise in significance of Delenn. Infection the first indicator of the prior Shadow war. Mind war the future evolution of man and the existence of the First Ones. Sinclairs kidnapping to find out what happened on the line is part of his future importance.
Those are just major arcs in the first ten episodes. Minor arcs exist and help form background.
You do realize that was just one example. B5 is full of continuity where things in earlier episodes link to later ones. Earlier shows lacked that to a great degree.
Soap operas did it all the time.
Which I referenced.
Different eras. in the TOS era what TV show had a consistent on going story line? Soap operas. By TNG others had been added (BSG for example). By Enterprise even more like Stargate and Babylon 5, Firefly.