On October 16th, the release-1.4 release branch will be cut from master. After this point in time, any changes that are needed in the 1.4 release will need to be cherrypicked from master to the release-1.4 branch.
Please see https://github.com/istio/istio/wiki/Istio-Release-1.4 for all details about the 1.4 release, including information about how to cherry pick changes.
Thanks