How to diff between git tag and branch

Thanks everyone for the leading suggestions. The issue seems to be that I didn’t have a local branch 1.4. I could achieve what I wanted (i.e. diff between the tag and the branch head on the original Git repo) in a couple of ways:

1. Switch to it (with git checkout 1.4), then I could do git diff v1.4.16
2. Refer to it on the remote: git diff v1.4.16..origin/1.4

Both achieved what I wanted.