What bit shall I use to encourage a bend on a young horse?

I am currently riding my welsh d mare (4yrs) in a loose ring sweet iron bit with bronze rings, I'm not sure on the name. I am unsure on how to encourage a bend using the correct equipment while she is young. I ride her correctly and work her from underneath and ensure she uses her behind but still she only bends when she feels like it in walk. I have been told to use double reigns and pelham but I would have thought that would be too strong for such a young mouth? Perhaps a bungee? Please help, all answers welcome and the best and most polite will be given five stars. Thank you.