The £4k quoted will only be on the assumption that you pass in minimum hours. You will therefore, need to find their normal hourly rate and budget for extra hours in case you don't get in 45 hours (JAA figures).
You will also need to factor in aircraft availability and serviceability and yes, even in the States, weather! So if a school claims they can get you through a PPL in 3 weeks, I'd budget on accommodation for 6 weeks to be safe. I may also want to factor in a loss of earnings figure - even if you're between school and uni, you could still earn some money here!
If you do go to the US to learn, I'd advise doing your theory exams in the UK first (whether FAA or JAA).
Cheers
Whirls