Grargar: If it was solely up to Ubisoft, then I believe that Myst 3 and 4 would have already been available here and/or on Steam. Unfortunately, since Cyan owns the Myst franchise (but Ubisoft retains the rights to those two specific games), the usual licensing headaches pop up in the horizon. Cyan themselves have been commenting every once in a while for 5 years that they have been in talks with Ubisoft into rereleasing the games, but nothing has come from it so far.
Well, I meant s/Ubisoft/the rights owners of the game/ of course. :)
If one looks on the wishlist entries you'll see people complaining to GOG like it is GOG's fault the game isn't here and GOG isn't doing their job blah blah blah, but of course any rational person knows that GOG doesn't get to just randomly hand pick games they want to come here and they magically show up a week later because GOG has authoritarian control over all companyies and their games. It's always up to the rights holders whoever they are, and only in GOG's ballpark if a game is offered to them and they reject it for some reason.
So like any game, it's ultimately up to the various rights holders of the IP, whomever they happen to be, but Ubisoft is the publisher and Cyan is the developer on the boxed copy I have here at least. Some combination of them are likely why these games aren't here. :/
Dzsono: Unfortunately, I know nothing about the Myst series and its compatibility problems, but I did pop my head in to see if anyone directed you to [url=http://pcgamingwiki.com/wiki/Myst_III:_Exile]Myst 3 @ PCGamingWiki[/url] for a potential fix. Except the game page is suspiciously empty! If you manage to find out exactly what worked for you, please add it to the page, or PM me with the fix and I'll add it for you :D
Yeah, I saw that page and did a double take at the claim "works well on modern systems" as you can find an endless streak of people trying to figure out how to get it to work on Vista, 7, 8.x, 10 and even many people on XP. So it seems that page is more of a placeholder than useful at the moment. :oP
Not sure if I'll have useful info to add there though unless I can figure out a fix that is reproducible which works, which is looking unlikely at the moment. :/