It is actually pretty simple. Mercurial automatically assigns a _local_, sequential revision number to every commit in your repository clone. In fact those revision numbers almost match the numbers in your examples (Commit 1 would have revision number 0, Commit 2 would be revision number 1, etc).
So in your example you would do:
"hg update 2" to checkout Commit 3, and
"hg update 3" to checkout Commit 4
Note that these revision numbers are _local_ to your repository. That is, another clone of the same repository may have different revision numbers assigned to different revisions. The revision numbers are assigned in the order in which commits have been added to that particular clone.
You could of course also use the revision id (i.e. the SHA1 hash) to identify them. You do not need to use the whole revision id, just the part that makes it unique, usually the first few (e.g. 6-8) characters of the hash.
In addition Mercurial has a very rich mini-language to query and identify revisions in your repository. These queries are called "revsets". Most mercurial commands accept revsets wherever they would need to receive a revision identifier. With it you can identify revisions in many ways such as by date, by belonging to a given branch, commited by a certain author, containing a certain file, etc (and any combination of those and many others).
Finally, if you use a good mercurial GUI (such as TortoiseHg) the whole thing is moot because the GUI will show you the whole DAG and you can just click on the revision that you want and click the "Update" button.
I actually find the ability to create these "anonymous" branches really useful. Naming things is hard so I find that coming up with a name for each new short lived branch is annoying.
So in your example you would do:
"hg update 2" to checkout Commit 3, and "hg update 3" to checkout Commit 4
Note that these revision numbers are _local_ to your repository. That is, another clone of the same repository may have different revision numbers assigned to different revisions. The revision numbers are assigned in the order in which commits have been added to that particular clone.
You could of course also use the revision id (i.e. the SHA1 hash) to identify them. You do not need to use the whole revision id, just the part that makes it unique, usually the first few (e.g. 6-8) characters of the hash.
In addition Mercurial has a very rich mini-language to query and identify revisions in your repository. These queries are called "revsets". Most mercurial commands accept revsets wherever they would need to receive a revision identifier. With it you can identify revisions in many ways such as by date, by belonging to a given branch, commited by a certain author, containing a certain file, etc (and any combination of those and many others).
Finally, if you use a good mercurial GUI (such as TortoiseHg) the whole thing is moot because the GUI will show you the whole DAG and you can just click on the revision that you want and click the "Update" button.
I actually find the ability to create these "anonymous" branches really useful. Naming things is hard so I find that coming up with a name for each new short lived branch is annoying.