Crystal, have to say that I think your DD is at the height of a bossy phase, this seems really really common with girls; I posted on a thread the other day where a mum was asking "are all 4 yr old girls bossy?"; concensus was yes they ALL are!! It is a phase though, as everything is I bet you she will become less controlling over things as she gets older. My neice was extremely bossy at 4, really quite insufferable, poor girl, but she's six now and a delight.
As I'm sure you're already trying to do, don't get into arguments and debates with her. If you are saying no to something, explain why, repeat once, then simply tell her that you are not talking about it any further (and stick to it!)
Once with my ds I had to do this, my refusal to discuss it made him mad and he followed me around the house ranting at me, so I went to my bedroom and shut the door on him for a couple of minutes, calmed myself, and thought about what I would do with him when I opened the door; finger painting on our patio doors was always a favourite activity of ds', so I just came out planning to do that, got started, and eventually ds couldn't resist joining in and we moved on.
It really, really is key to be able to move on. I know that feeling of burning with internal anger but breathe deeply, visualise your glass of wine and mag waiting for you after bedtime, and MOVE ON.
Your dd as a child, as all children, is reactive and will endlessly react....it's up to you to give her different stuff to react to, to stop the arguments dead so that she can move on.
If she's completely changing the rules all the time, let her do things for herself - just tell her that you can't know what she wants done as she changes the rules, she must show you....