I'm sure I skimmed a topic on this on this forum but it was before I was testing IPB4 so I didn't take much notice and I can't find it now 

My members use the [ topic=xx] and [ post=xx] features quite a lot. It seems like topic still works in IPB4, but post does not. Is this intentional?

Yes, it is intentional: http://community.invisionpower.com/topic/407509-forum-post-numbering/ 

You can still share post location using share icon in upper right corner of every post.

Yes, it is intentional: http://community.invisionpower.com/topic/407509-forum-post-numbering/ 

You can still share post location using share icon in upper right corner of every post.

​I don't think that topic is related to the issue raised by AutoItScript.

He is talking about the BBCode tags that we had in IPB 3.x.x to generate links to other posts and topics from their ids.

It does actually.  "topic=" still works "post=" doesn't, and the new way to do this is to use the share link and embed it in the post

[...] the new way to do this is to use the share link and embed it in the post

​It's not exactly equivalent.  A lot of people were already using links before, but the BBCode was more "abstract", and it would allow you to change the URL format of your community (or even change the domain) without getting broken links, for example.

Unfortunately, it seems the trend is for IPB to move away from abstract markup to HTML, without understanding the downsides of the move.

 

PS: And I think the link Morpheus NS provided discusses the removal of the posts numbers, which means people cannot refer to a post by its number.  But this number has nothing to do with the post id.  (Actually, I never liked to use post numbers because they change when we delete posts, which means they are not a reliable identifier.)