Sounds like you are right on top of things :)
I had the same situation a few years ago. I'm not a trainer, but I got a mare out of the field who had only had a few rides on her (over the course of several years) before I got her. Luckily, yours is young so you have a distinct advantage (mine was almost 8 when I got her and hadn't been worked a day in her life). My mare has a VERY hard time cantering as a result of this. Do you canter her on the longe? At 4 years old, I would get her working well in the canter without a rider, so that the transition to carrying a rider at the canter is easier.
Other than that, obviously more trot work under saddle, and teaching her to go off the bit would be your first priority. Good luck and have fun!