I've been thinking of the possibillities of a volume 4, and after thinking about it and looking at the episode guides at Muppet Wiki, I actually feel less excited about a potential volume 4 than I did the existing ones (while I feel a little more excited about a set of the season 21-25 premieres, for a number of reasons).
Don't get me wrong, I'll probably still buy a volume 4 on the day it comes out, but most of the known contents don't excite me as much. Sure, it will be great to finally know what's in the season 16 premiere (for the sake of the set I'm hoping it's like a "gold mine"* for fans and collectors, though it'll probably be less than the gold mine I'm hoping for) and to know what Noggin cut from episode 2096. I do want episode 2096 on DVD, and the Miles' birthday episode looks interesting, and it'll also be great to have Jazz #8 on DVD, but many of the best clips in the episodes have already been on video (especially in past Old School volumes). I'm not too excited about seeing the first part of Maria and Luis' honeymoon, or the trip to the zoo, though they might be better than I think (some of the "street stories" in volume 3 are a lot better than I had expected before seeing them).
Of course there'll probably be some really great bonus features, and I'm sure I'll be happy with many of the bonus clips. Hopefully the bonus clips will include some clips that we haven't even heard of (it seems the late-1980s had less "holy grail" inserts that we are aware of), in addition to clips that should have been released on video years ago.
Thinking about it, I think I'd like there to be a collectors set full of inserts from seasons 1-15 in the summer, and an Old School Volume 4 in october or november. I wonder how likely Sesame Workshop/Warner Home Video would release two sets for the adults.
*In this case by "gold mine" I mean it'd have previously-unreleased Sesame Street News and Monsterpiece Theater clips (I don't think there are many unreleased MT clips from before 1984), maybe a Super-Grover or Waiter Grover segment, some clips from the first season (I'm hoping a non-2 Jazz Numbers and Baker film, and any Muppet clip from that year), and appearances by most if not all of the following characters: Biff, Sully, Bruno, Dr. Nobel Price, Don Music, Forgetful Jones, Sherlock Hemlock, pre-Kevin Clash Elmo, and maybe Deena and Pearl, Aristotle, and Leslie Mostly (those characters were discontinued by then, but I don't know if they continued to show their inserts). I know it's wishful thinking, and probably far-fetched for the episode to have included ALL of those things.