If she holds your hand and isn't a flight risk, I really don't understand why you'd put a 4 year old child on a lead, poor little sod.
For dc with different needs and who struggle to understand not bolting off, I can get on board with as it keeps them safe. Safe from usual outside hazzards, like cars and rivers etc
But to use them in case a stranger snatches your dd while you are buying an ice cream, I think you need to have a little rethink on it.
In the kindest possible way, you have a perfectly lovely 4 year old, who behaves and listens to you and holds your hand, yet you're still going to put the poor little mite on a lead (sorry reins, but same thing).
What incentive does she have to staying with you and listening if she still has to be tied to you all the time?
If you feel safer, maybe take it and just reiterate to her to stay with you otherwise you'll need to use the reins on her. I mean its a threat/punishment, right? I can't see how any person, child or not, would be happy being walked around on a lead when they know and have shown they are capable of not fleeing the scene.