If you are simplistically telling me a rate of 4 lb a week is in every case unhealthy then sorry we will just have to disagree. It's not average, but is not uncommon at all. As for whether it stays off or not being a rarity
it's an achievement you either fail at or you achieve - that is the failure of the person's overall life skills over and above anything else. If you are looking for long term good weight/health then do the work and research in getting to that - if you have a lot to lose and don't mind losing it over a matter of years, with no rush regarding the meantime effects on your health, by all means go ahead with slow weight loss, but at the same time independent of that short term dieting for weight loss is effective and shouldn't be scoffed at or completely written off. One size certainly does not fit all.