I have a task that will take 4 hours to complete, but it will be done over a 1 week timeframe. I can't get project to accept fixed start and end dates with a time estimate of that is only 4 hours. How do I do this??????

Also, how do I enter time against a task? What I mean is I plan a task lets say for 8 hours and assign a resource to it. If it takes 12 hours to complete, how do I enter that time without messing up the planned hours of 8?