My opinion Pagefan is to wait until you have an offer and then ask to switch. You don't want to hassle them while they are reviewing your application/busy with admissions. I imagine the entry conditions aren't too different from the 3yr to 4yr so if they gave you an offer for one they will likely be happy to switch you to the 4yr course.
If the entry conditions are different then would you rather have an offer for the 3yr course, or no offer at all? Its safer just to wait for their decision and then ask nicely to switch after. That is why I believe it makes sense to wait.