On a Q&A site in Japan, I read an expression that as if comes from as (would be the case) if.
Are both about the same thing?
Is this claim historically true?
If so, what does "as though" come from?
Sense 4b describes as though but adds nothing of use.
I believe as if to be very old, possibly having existed in Proto-Germanic, because Dutch has alsof with the same meaning, and German als ob.
The conjunction as could be used to mean as if in English, at least between 1100 and 1800, according to the Oxford English Dictionary (but possibly in Old English too). An example:
In Middle Dutch, and possibly also in Middle English or Old English, of/if could be added to subordinating conjunctions or relative pronouns in what seems to be a somewhat arbitrary fashion. This is still possible in some dialects of Dutch, as in vertel mij wat of hij doet, [tell me what if he does], "tell me what he does". It can even be placed before a conjunction/pronoun, as in ik zie niet of dat hij daar is, [I see not if that he there is], "I don't see if he is there". Note that Dutch als normally introduces a condition here.
My hypothesis is that if could somehow be used in a similar way in other Germanic languages, like English, and that this resulted in something similar to modern as if / alsof / als ob already in Proto-Germanic. But I cannot prove it. It could also be a convergent development.