Actually a recent ruling by the ECJ might make these changes in some sense viable:
http://www.bbc.co.uk/news/uk-politics-30002138
This story didn't get nearly as much attention as it deserved on this site. I'm not entirely sure why, perhaps it was just never picked up on I guess, although I suppose I could have started the discussion myself.
One thing that story did show is that benefits tourism isn't confined to the UK, and a lot of the richer European countries have been suffering from it, albeit to a far lesser extent than is often believed. Indeed, until that ruling there had been a lot of decisions going against the German/ French/ UK governments on this sort of thing, and it was becoming a bit of a mess. The ruling above has the potential to set things straight.
As regards Cameron's proposals, I'd be worried that they are going a bit too far. Four years is a long time and could penalise someone, for example, who came here to this country with the legitimate intention to work, but had the bad luck to choose a job in a company that went under not long afterwards. Benefit tourism -- people deliberately exploiting this system -- remains a very rare phenomenon, so measures that can catch the unlucky along with the guilty ought to be avoided.