I suspect it sort of goes like this: DMG1 (or PHB1 etc) sells well, everyone needs it. If 75% of those people buy DMG2 it still sells fairly well, but by the time you get down to DMG3 with 75% of the people that bought DMG2 picking it up the book just isn't a money maker. Beyond that people coming into the game see all these books and they are confused. Either they wonder if they need ALL of them to play, or they wonder if DMG3 replaced DMG1 and DMG2. Either way they're a bit confused.
Isn't there still a book in the "sometime next year" category that has been discussed that is supposed to have character options and additional rules for this and that (crafting and whatever)? That seems like it is sort of partly "DMG3" kind of material. More like providing stuff that was in the 1e DMG, rather than the epic focused DMG3 that was originally proposed.
Honestly as far as PHB4 goes I think they just plain ran out of enough material that was generally interesting to a large enough base of players to fill such a book. What races would be in it? I mean PHB3 was already down to Shardminds and plant men. Yes, I'm sure people can list all sorts of things THEY would like to see in races, but the problem is no one set of them is interesting to enough players to sell the book. Likewise with classes, the iconic stuff is done.
Seems to me the current strategy makes perfectly good sense, switch to smaller cheaper softcovers that each concentrate on one narrow thing, like Heroes of Shadow. The book is cheap enough to produce and has a low enough price that it can make money even at modest sales, and at $15 even people that are only marginally interested are likely to still pick it up.
They can always do a 'savage species' type book for all the oddball races, a 'book of ritual magic' for those of us that like rituals, etc. The sequential core books just ran their course. I mean I general like to have every rule book, but when even I stop buying them, yeah, it's pretty much time to call it quits.