I am going through a similar stage with my 4 yr-old daughter. I used to be a real push over before this stage, but now I have to be consistent at all times. I recently read a book about counting 1-2-3, although it is not as simple as it sounds. The name of the book is 1-2-3 Magic by Dr.Thomas Phelan. The basics of the book are you need to count at a calm tone, don't talk your way through the count (i.e 1 i am going to send you to your room-2 you better listen to me i will send you upstairs- 3 i mean it last chance), on the count of 3 you need to follow through. Although hitting and swearing (as long as they know the meaning) is an automatic 3 and straight to timeout. My daughter always tried to give excuses at first, but now she knows I mean business (not to say we still don't have bad days, but things are starting to shape up). I hope this helps because I know the days when you might feel like you don't know what else you can do.
at 10:34 PM on Jul. 20, 2008