Hi J.. It sounds like your daughter does not make the connection between what you tell her to do and the natural consequences that may occur. Also, you say you've tried everything, but you haven't given us any sense of how long you've tried any particular thing. I get the sense that your daughter has already figured out that you're not sure how to exercise your authority and that given enough time you will just crumble. :-) It may even be that she feels she can't depend on you, as the authority figure, to meet her needs so she is taking it into her own hands to do what SHE thinks needs to be done.
So I would say that the first thing you need to do is think about the rules in your house. Try to explain, just for yourself, why you have the rules that you do. Ask yourself for each one "Is this important? Why? What will happen if this rule is not followed?" You must be clear in your own mind as to what boundaries must be followed for safety.
And then you must clearly explain to your daughter what these rules are and why each one is important. You can even make a little poster that you put on the wall, "Our Rules." Invite her to get involved with you by asking her what would happen if she didn't follow the rule, what harm might happen. And maybe she will point out that it doesn't ALWAYS happen. Such as with her wandering off unsupervised. "Nothing bad happened, Mom!" "Sure, this time!" She needs to understand WHY you don't want her to do this, and the severity of what may happen.
But then, you need to work with her to compromise, as the other mom suggested. If, for example, she can't just take off when you've told her "no" what can she do instead? Can she invite the other girls to come in and play? Can she arrange a supervised playdate at their house? Try to figure out some way that her need and desire can be met, reasonably and within the framework of your rules.
You can even make a new poster together that includes these other options. Since she is just learning to read, you can make little pictures to serve as icons to help her remember what each one says. And don't be surprised if she thinks there should be some rules for you too. There is no reaon that some household rules shouldn't apply to everyone. (At our house it was "no eating messy foods on the couch".) But limits imposed on you should be appropriate and respectful.
As a teacher working with parents, the three biggest causes I see of disobedience are:
limits are not communicated clearly and consistently, including what the consequence will be if it is broken,
parents don't educate their child about WHY a rule is important (and why the consequence is being imposed)-- or don't do so in a way the child can relate to and appreciate
parents don't acknowledge the need that drives the child to the behavior and provide an appropriate way for the child to get their needs met.
Obviously parents don't do this because they are "bad" but because they are lacking in information themselves. Many of us were parented by a generation that followed the formula "Do it because I SAID so! <whap>." And we know that didn't really work, but we're not sure what else to do. We need a model for how to be authoritative -- insted of bullying or wishy-washy (or as often happens, a little of both).
Some books you might find helpful in this area are "How to Talk So Kids Will Listen: And Listen So Kids Will Talk"" and "P.E.T. -- Parent Effectiveness Training".
You can find them at your local library or bookstore or order them online here: http://www.earthskids.com/behaviorbooks.aspx