I'd like to add that 4 and 5 year olds don't always understand talks - in fact I think they understand A LOT less than adults think. My point is this, have the talk, but in all circumstances politely refuse the awkward requests one or two times as is proper, then ignore her after that. Then on a totally separate interaction put her on timeout or discipline her in your normal way if she continues to focus on, bring up, and badger you about the belly button. Don't make the belly button the focus of anything, instead focus on her behavior, specifically not respecting your response.
This will teach her that when she is a grown up woman she can expect men and others to respect her wishes to not do the same thing. You are modeling for her what she will be when she is older.
How you choose to act, what you focus on is going to be how you teach her. Your talk can help her understand, but the teaching comes from the actions.