I recently evaluated GameTap and wrote an article about it, from an
adventure gamer’s perspective. If you’d like to read it, the
article is here: http://www.adventuregamers.com/article/id,608/
I wouldn’t be surprised if they add SQ4 (and 6) at some point.
The ones that are up there right now weren’t added in order, and the
games are added randomly, so it’s very possible the whole series will
be up there at some point. GameTap also has several PDF manuals
stored on their site (this has changed since the article was printed),
some of which are for King’s Quest. So it wouldn’t surprise me if
the KQ games get added to the service. Someone reported on the
Adventure Gamers forum yesterday that he saw a ‘coming soon’ ad for
Quest for Glory II, also.