In my git repository, I made 5 commits, like below in my git log:

commit 4f8b120cdafecc5144d7cdae472c36ec80315fdc
Author: Michael 
Date:   Fri Feb 4 15:26:38 2011 -0800
commit b688d46f55db1bc304f7f689a065331fc1715079
Author: Michael
Date:   Mon Jan 31 10:37:42 2011 -0800
commit b364f9dcec3b0d52666c4f03eb5f6efb7e1e7bda
Author: Michael
Date:   Wed Jan 26 13:33:17 2011 -0800
commit 4771e26619b9acba3f059b491c6c6d70115e696c
Author: Michael
Date:   Wed Jan 26 11:16:51 2011 -0800
commit 6e559cb951b9bfa14243b925c1972a1bd2586d59
Author: Michael
Date:   Fri Jan 21 11:42:27 2011 -0800

How can I roll back my previous 4 commits locally in a branch? In other words, how can I create a branch without my latest 4 commits (assume I have the SHA of that commit from git log)?

To create a new branch (locally):

• With the commit hash (or part of it)

  git checkout -b new_branch 6e559cb
  

• or to go back 4 commits from HEAD

  git checkout -b new_branch HEAD~4
  

Once your new branch is created (locally), you might want to replicate this change on a remote of the same name: How can I push my changes to a remote branch

For discarding the last three commits, see Lunaryorn's answer below.

For moving your current branch HEAD to the specified commit without creating a new branch, see Arpiagar's answer below.