The way I understand it is this:

4 years ago today Q posted "Castle LOCK" along with a photo of a watch that shows exactly 4:49pm on it.

Not only doe the numbers 4+4+9 = 17, but the Queens coffin was officially lowered into the royal vault at 4:49pm. What are the chances that that is a coincidence (same date, same time) vs the other perspective that the Q Team and White Hats are in control of this movie script?

So, it's a form of Q proof.

No one knows, yet, what "Castle LOCK" means, but there are some theories, such as she is officially locked in the castle, or that the castle can now be locked down. But "LOCK" is all capitalized. So it may be an acronym for something.

Hope that helps.

Edit: I might be wrong... I am not an expert at this.

Edit #2: There is no pic of the watch with that Q post!!! WTF??? Here's the post: https://qalerts.app/?q=3093024 I am pissed, because I sent this to some friends who don't believe anything about Q--and I ASSUMED that this was a Q post. So now I have to check everything that other Anons post here to make sure I am not being misled? This really sucks.

Edit #3: So, as you can see from replies below, it was explained to me that the date of "19" on the watch is the extra tie that makes the 4:49 relevant to today even though it wasn't on a 9/19 post by Q, if that makes sense. So, still a powerful proof... but I still wish that the watch photo had been posted on the 9/19/2018 Q post, which is what I had first thought was the case. Thanks, Purkiss for the explanations.

Edit #4: In case you are reading this... I also just learned that the capital letters of the 2214 post spell CLOCK, which points over to the post with the watch photo. So, another strong link that suggests that this was a two-post puzzle to put together regarding the lowering of the Queen.