A branch is nothing but a pointer to a certain commit. As @LethalProgrammer said in the comments, the HEAD of your current branch is the commit it points to. Origin now is something completely different, it's the default name for the remote repository (i.e. the repository on the server, where you pull from and push to).
When you're on branch v4000 with Z as it's HEAD (your current state as far as I understand), you can just create a new branch v4100 and reset the v4000 branch back to Y.
- On the command line, this looks like this:  - git stashstash any uncommited changes
 - git branch v4100create branch- v4100with- Zas it's head
 - git reset --hard Ywith- Ybeing the commit' s SHA id: reset branch- v4000's- HEADto commit- Y
 - git stash poprestore previously stashed changes, if any
 
- You can also do this via VSTeamExplorer, but I don't know how to stash there, so be sure that you have no uncommited changes before you try this: - Select - Branches->- New Branch, enter- v4100as branch name, uncheck- Checkout branchand click create. This creates branch- 4100with commit- Zas it's- HEAD.
 - Then, still in the - Branchestab, select- Actions->- View History. In the opening window, right-click commit- Yand select- Reset->- Delete Changes (--hard).
 
EDIT: As always, be careful when using git reset --hard either way, since it can potentially result in data loss.
Just this description should be enough to convince you that using git via command line is cleaner and it's much easier for people to explain stuff via native git commands. And,as @axiac recommended in the comments, reading the Git Book is definitely worth the time.