To be fair regarding the Manu incident, he got two 5 match bans IIRC, one for the original incident and the other for having a couple more goes afterwards. There was plenty of build-up in that and previous games that probably should have been dealt with differently, but the ban for the incident in isolation was probably correct.
Back to Greg Bird, whether 4 games, 5 or 6 or more is correct, the main injustice was that the ref failed to deal with it properly on the day. The ban confirms that a red card should have been shown rather than a yellow, and it's not exactly as if there was any subtlety in the incident that means that the officials (between them at least) shouldn't have had a clear view of what happened.