
Four year olds can really be quite expressive, bright and lovely. May I ask if you've every simply discussed all these things with your DD? If so, what's her take on it? Four is plenty old enough to recognize that peers are no longer in diapers. What does she have to say about that? There must be (or should be) some way to stimulate a desire on her part to match their behaviors - this is 100% natural in kids, and a big part of how they learn. It really won't be long before other kids notice that she is still in them, if they haven't already and that will likely not be a positive experience.
And IMO, don't be afraid to
just tell her it's time to be out of diapers. Not unkindly, just very matter-of-factly. Put whatever practical steps you choose in place, but don't imply training is a choice - it's just time now. Try not to worry about traumatizing her - you obviously provide a very loving home. First-time parents want to do everything just perfectly and that's just not possible. Give yourselves a break there.