Possiblely unbalance with a rider at 4, but if you cannot get her to trot on the lunge, then it doesn't surprise me that you can't get her to trot under saddle.
She isn't hurt, right? Will she trot out in the field? Would she trot to her feed bucket?
How badly do you want to trot? She is rising to the level of your expectation, not what you think you want but what you are saying you will be satisfied with, saying with your actions. So if you really want her to move out, you MUST say so with your actions, and accept nothing less. She will rise to your new level of expectations. She may complain, she may swish her tail, jump around, buck a little, you name it, but she CAN and WIll trot if you require it.