i think the vassal heroes are different from the 4 Heroes. I mean when you count this world's hero weapons there are way more than 4. there's the Scythe, the book, the mirror, the airship, the katana and then there's the fishing pole.
I never read the LN/WN, but i think the Vassal heroes are unique to this world, because there's no mention of them during the previous world arcs. These vassal heroes seems to be able to move freely between worlds. Like during the spirit turtle arc, they can move to that world even when there's no swarm going on.
The 4 Heroes doesn't seem to be able to cross world. The Shield Hero managed to do that because of the spirit turtle and even then he received heavy penalties/restrictions.