Hi @oscargoldman,
Gitlab 11.4 is compatible with Mattermost v5.3, so you would need to upgrade to Gitlab 11.5 to be able to get Mattermost v5.4. The chart in this link shows which Gitlab version is compatible with which Mattermost version: https://docs.gitlab.com/omnibus/gitlab-mattermost/#upgrading-gitlab-mattermost.