Originally Posted by VydorScope
Not sure what you mean here...
He means an omnibus is just a collection of separate works in a single volume.
It doesn't mean all those works are a series of any kind, or even are necessarily related, beyond whatever reason someone decided to publish them together.
So while a "Collection of the first four works", if published as a single volume, and not, say, a box set of the four separate volumes, would indeed be an omnibus edition, it doesn't seem to address the original question.
The term does not convey the connection between the works that "trilogy" does.