After git installation - I tried to clone an existing p4 branch, but it failed with the following information:
c:\P4_GIT\DT>git p4 clone //depot/CTAT/Windows/OneP/
fatal: 'p4' appears to be a git command, but we were not
able to execute it. Maybe git-p4 is broken?
I've had more success (on Linux admittedly) downloading one of the git-p4 branches from github and calling it directly (git-p4) rather than through git p4.
Maybe that would help you?