Here’s the Fourth Way in a sentence: there are things that can only be said perfectly and precisely of a being that all recognize as God. St. Thomas lists three or four: good, true, dignity and existing, but he gestures in the direction of many more.
Mistaken or imperfect notions are used analogously to the correct or perfect. Malaria said of “A disease caused by bad swamp air” and “fever caused by a pathogenic protozoa that invades red blood cells” is used like this, as is heat when said of (the supposed element) fire, phlogiston, and mean molecular motion or “dropped the Hiroshima bomb” when said of Truman and the Enola Gay.
Imperfect or imprecise notions need not be separable from some subject and can even belong to it essentially or necessarily. While we can have malaria without swamp air or swamp air without malaria, it does not follow that we can have fire without heat or a dropping of the Hiroshima bomb without the Enola Gay (or some plane like it). But we can still have to distinguish these sorts of essential or necessary things into a primary and secondary.
Though I’m speaking of analogous uses of terms and how things are known, they are shorthand to make a metaphysical point. True, the drift of the argument is this:
Mistaken and imprecise notions are said analogously to the true one.
All notions that are separable from some subject or secondary to it are imprecise.
But the point is not to talk about how something is known but about the reality that is being said of the thing, and when considered this way it is hard not to notice that existence is said separably of any natural substance. On a substance-ontology, where everything traces back to substance and neither the universe nor matter nor forms/laws in mathematical abstraction are substance, it’s clear that natural substances exist only derivatively from a supernatural deus. But the contemporary mind has a great deal of metaphysical confusion about things like universes, physical laws and conserved quantities which makes us less able to conclude to divinity. We recognize easily that natural substance depends on something everlasting, uniform, and even abstract-intelligible but we hesitate to call it “God”. We run the same argument on the idea of “dignity” and think it should conclude to us.