Speaking of flashbacks in series 4: flashbacks seem more and more plausible with every piece of new data -
at least that’s how I feel it, because mofftiss need to show the
audience they were implanting the idea since a long time, and that
inside the show John and Sherlock were in love since the beginning, that
everything they did was directly consequent to their feelings - so a
flashback seems an appropriate tool.
Also, it will not be the first time they use it, they used flashbacks to show they implanted the idea of Mary being an assassin, in little details most of us missed at first watch until we got the flashbacks.
There is a nice probability that they also used it before to make us familiar with the concept: we’ve shown you something that was subtle enough for you to miss it, now we’re going to show you the trick and make you realise it was there.
This is what they’ll do on a larger scale, and now that the audience is familiar with this tool, they expect people to open their eyes wider. The use of this is supposed to make the audience think: is there something else I missed?
This is why I believe there’ll be flashbacks in series 4, and that the Six Thatcher will be one of them, maybe one of the most important since the case calls back to a period in Sherlock and John’s lives when they realised they were more than friends, and that they where lying to themselves by pretending these feelings were not there, thanks to Irene clear-sightedness.
This was a pivotal moment in their relationship, so going back to it is pretty meaningful, because it’s there that the global audience should have seen the clues that it’s not “a cheap gay joke” (quoting Gatiss in TGG’s comments), but the whole point.
I’m also pretty sure there will be flashbacks to Moriarty and Mary, to the link that may exist between them. We’ve seen a glimpse of Mary in Morocco - it’s not unreasonable to think this is a moment from her past. We also suspect she was one of the sniper at the pool in TGG, this theory is supported by some solid elements, and if she is indeed involved in this (as Moran or else), there will be some explanation of it.
Finally, flashbacks seem also an appropriate tool for the Redbeard/The Other One mystery. This is something from Sherlock’s past, and we know children have been cast to play Sherlock and Mycroft so to me this is actually the most evident way the flashbacks will be used. We just need to see what happened, so the problem is solved. And as I said here The Six Thatchers may be an episode based on this problem, since “Maggie” is the password.
So, we have three excellent reasons for flashbacks to be used. It can seem maybe a bit “flashback ex machina” when we see data and whisper “flashbacks”, but actually, I think we can logically expect quite a lot of them.