Quite. I know I sound like a broken record, but I honestly don't think we are going to get anything for at least 5 years or more, otherwise why draw attention to it? TFP was an ending for a reason.

http://www.digitalspy.com/tv/sherlock/feature/a819122/sherlock-season-5-air-date-cast-episodes-news-and-everything-you-need-to-know/

Nothing definite or new, but it gives me optimism.

Well, I can imagine we get a series 5 in about 5-10 years (I expect more towards 10).
I think they now completed their version of young Sherlock Holmes and Dr. Watson. But I think all of them would like to explore the more matured and wiser Sherlock+John. There are lots of other adaptations already, but they would still do it different and in their own way. But this needs some time for everything to settle down. be it the minds of the audience, Ben's and Martin's busy plans, the writers fresh and new ideas, maybe even the passing of time in reality linked to the story (for Rosie being old enough to go to a boarding school...). It would be another style of BBC Sherlock, but it would fit, because they never wanted to do the same thing over and over again.
I guess we would have to wait nearly ten years, if we get anther series.
 

Anything is possible.
Obviously one does have to have an eye for the older cast.
But at the end of the day, the show only really needs Sherlock and John.

I'm going to have to say it seems like the person who wrote this is really reaching. The message from Moriarty included in there seems to me that it's like Moriarty's comment on his reappearance in The Final Problem, not hinting at another episode.
Now, I don't know why anyone would joke about secretly dropping a fourth episode, but I still doubt that it actually exists. I mean, I would be OK with being proved wrong, but it doesn't seem like solid evidence to me.

There is clearly no episode 4. 
3 episodes per season is the system.
There could be another special.
But at the moment, 4 x 3 episode series is what we have.