Adorable! Let her do more if she wants. There was a 4 year old kid at my old lesson barn that rode a Percheron mare. She looked like a tick up there, but was an awesome little rider, and was cantering 2' courses by the time she was 6.

If you are able, a good way to introduce kids to trotting and cantering is to sit up behind them, that way you can hang on to them and steer/be emergency brakes if need be.