Getting the "message" given a branch designation and conversely

I am learning how to use git. I would like to know how:

Given a branch's designation, such as "master~4", how can I see the message I furnished when I created the branch using "git commit"?

Conversely, given the message I furnished to "git commit", when I created a branch, how can I see the branch's designation?

Context:

    Operating System: Red Hat Enterprise Linux Workstation release 6.8

    git version: 1.7.1

Thank you, very much.

    Norman Shapiro