Just wait a moment.
A fourth expansion would make this the most expensive computer game on the market, seeing as fixes, repairs and patches which are necessary to play the game (and changes to the UI which were crucial) came in expansions DLC.
Milk it, but not that much, folks.
I adore this game, but, say, $100 not-sale price with all expansions for a computer game would be pushing it.
I think it depends on their perception of the sales market. If they're primarily selling to people who have already bought up to 'Shadows', and they think there's a business case for selling those people another 'expansion', then why not go for it?. If they're trying to sell people the whole 'package' it'll be much harder, especially when the first game was released 3-4 years ago. So I can certainly see there may be a case for releasing another expansion. I'd buy it, but then I go around wearing a small piece of plastic with a vacuum operated end bit attached to my forehead. (Only kidding 'Matrix guys'.)