Today I was looking for a way to link to a post in the same topic.
The 
icon, when clicked, shows post #4, so I tried: using #4 and post#4, only to get frustrated, but discovering that the # character pops up categories to link (which I read about extensively since, in meta.)
Now, I find this quite inconsistent, and would like to propose ways to consistently address internal links that matches the actual URL format, or already used patterns…
Generic Patterns
Using colon as a prefix or separator would be nice, but it already autocompletes to emoticons and that could become messy. But consider: :4, :10:4, and 3:10:4 (with optional colon prefix) to link to post number 4 in the current topic, post number 4 in topic 10, and post number 4 in topic 10 of category 3.
Using / instead might work, but then it should match the URL, e.g., /c/3, /t/10, and /t/10/4.
The “quote reply” to a post uses keyword colon number, e.g., post:4 or topic:123. This can be very useful is there’s a way to show these IDs with every post (e.g., with an element title or tooltip).
In general though, #post_4 would use the actual anchor in the HTML. #4 would be great too, but it conflicts with the (current) category shortcut (BTW, what happens when you have a category name matching a tag name?)
Both solutions though are problematic for i10n since they impose English words.
The shortcut to a post in the current topic should make category and topic implicit.
Since topic IDs are global, .. can be useful, as in .[optional topic ID].[post number], e.g.:
..4to link to post #4 in current topic.10.4to link to post #4 in topic 10
