Sarenco - thats an interesting thought, but I think part of the issue is that many landlords would traditionally at times like this raised rents at whatever they felt the market could bear, and risk the tenant leaving on the grounds that they could get another. I recall that at the height of the boom from 1999 to 2006, increases of 15-25% were quite common, and I think there was a legitimate fear of large numbers of tenants being priced out of their homes over a couple of years.
That said, there is a legitmate case for landlords who have been charging well below market rates to be permitted to increase by more than 4%, because someone who is currently charging, for example 700 per month is going to have a lower actual increase than the landlord charging 1100 a month. Its complicated.....