Uhm actually you are wrong. Myst 3 & 4 were developed by Presto Studios and published by Ubisoft - Cyan hold the IP rights.
Uhm, I'm aware that Myst 3 was developed by Presto Studios (Myst 4 was developed in house by Ubisoft). Just because Cyan own the Myst IP doesn't mean they have full ownership of all the games in the series. Despite Cyan owning the Myst IP, some deal needs to be struck with Ubisoft, the owner of those games.
I don't think anything I said was wrong. Cyan is the publisher of Myst on GOG, Ubisoft would need to publish 3 and 4.