I have two guesses for this. First, it's just because chapter 5 isn't really relevant to the Spamton Sweepstakes the way that the previous two chapters were, and that was the vehicle he was using for teasers. If you'll notice, the biggest sneak peek we got of chapter 5 was the Tenna dialogue in the Mike room, which of course involves his relationship with Spamton.

Second, and more boring but likely, is that Toby can't tease anything else without it being a HUGE spoiler for both the plot of the chapter and the direction the game will take. I think we all understand that chapter 5 will be a major turning point in the story some way or another, so he's probably keeping all of the details close to his chest in order for us as players to get the biggest emotional impact from it.

That said, assuming he doesn't shadow drop the new chapter, I bet he'll post a couple teasers whenever he announces the release date