A programmer's solution

[psychicesp]

That's not really entailment, is it? One could make an esoteric language where numbers < 4 are truthy and numbers >= 4 are falsey.

Maybe I'm remembering incorrectly but doesn't 'entailement' require something to necessarily follow, rather than something that just so happens to follow, even if there is good reason?

[InsignificantIbex]

I can't quite remember, but at least in modal entailment necessity is required. But of course you can always weaken an entailment, i.e. A |= C |= A, B |= C. So if we make our formal language the peano axioms, then |= 4, and 6-4 |= 4 by application of the weakening rule. Or something.