Maybe I'm naive, but I think that's probably it - there's no decision to stop, so they haven't announced it.  But at the same time, no plans for the next series.  It's sad to admit, but as time goes on, I feel it's less likely that there will be a S5, and the team will just keep moving on with other things and not getting round to it.  As S4 wasn't so well received, it might even be the best decision ... I just don't know, maybe it has just gone out of fashion for the moment.  I still love the idea of them doing a sequel many years from now, so I'm still going to hold out hope!

Indeed!
But I never really bought the no-time argument. The show was not a stopgap, something you do when there is nothing else available. We know that Peter Jackson even postponed Hobbit filming so Martin could do Sherlock. 

Yes. They all are interested in multiple things, which is normal and healthy (not to mention good for their careers, if we have to take that angle).