Dusk and I commented on it:
For those wondering about GOG also releasing SH2 and SH3 - it makes strategic sense to just release SH4 first, since it would be the first Silent Hill game getting a digital PC release. That alone will enable it to swallow up more immediate sales simply because it fulfills a demand no matter what.
If SH2 and SH3 had been released simultaneously, many people would have skipped buying SH4 because it's seen as the inferior game. Now people will simply buy SH4 out of principle or over a decade of thirst, and then when the all-time classics in SH3 and SH2 are released, everyone nad their mother will pick it up.
So the SH4-only release makes sense sales-wise. GOG might also be looking to do some kind of Halloween type of promotion anyway when they want to release SH2 and SH3.
I have a suspicion they have something planned for SH2 & SH3. I think SH4 was prioritized first since people might've ignored it if the other two came out first (SH4 is often slept on unfortunately), though I'll also assume if they want to do something with them they may be considering the route to go with them a bit heavier as they're more beloved by many (plus complications with voice work, past port stuff, maybe they want to do something fancier with them, we know due to the HD Collection they're missing the source codes of the game, maybe need to track down the SH3 voice cast?).
I think there's a few reasons Silent Hill 4 going first makes sense, but I think SH4 coming to GOG is more a sign they're considering options with SH2 & SH3 more than they're skipping them. Of course, to see.