I have booked a three-week trip to California, on the Visa Waiver Program. I find it is a bit too short, so I want to stay 4 weeks instead of three weeks. Will that give me problems when I reenter the States in the future, because I didn't keep my promise of staying three weeks? Are there certain actions I have to take to let the immigration know that I will stay a week longer?
For an ESTA, the conditions require you to reapply for it if there are major changes to your status (like a change of name or citizenship) but not for minor changes, like extending your vacation by a week. You don't need to do anything and will have no problems.
Obviously you need to comply with the VWP rules (e.g. not stay longer than 90 days), and tell the border control officer on entry the truth about your vacation (i.e. it's 4 weeks). But you should have no issues at all.