Do you think Fallout 4 will be available on GOG ?
Fallout 4 will come to GOG in the future. It could be tomorrow, it could be in 5 years time.
I really think GOG will eventually offer Fallout 4.
Fallout 76, though, I would guess may not be something that GOG would offer at any point. I hate Fallout 76 anyway...
I hated 76 when it first game out, It's a totally different now than it was upon release though. I recommend playing it again if you have not played it recently.
Fallout 76, being online only, will probably never come to GOG. It's unlikely F76 will even work in the future, with it being very server dependent. We don't know what could be salvaged when Bethesda decide to drop the nuke on the servers.