I am looking at a 4 year old mare (4 as of May) that is butt high. I am wondering if she's just growing and should level out as she grows or If I should be concerned she might stay that way? She is Reining bred (Greys starlight, doc Olena, smart little Lena, Reminic all on her papers). 

There is a good chance she will remain hip high based on that breeding. She will likely grow a touch more in the next year and depending on her level of muscle development that may also play a factor. If she has not had a lot of conditioning she will develop a stronger and straighter topline in the next year too.

Thank you. Yes it sounds like she was started and in training for 10 months but most recently had just been turned out for the last several months so she's not well conditioned right now. 

How butt high is she? Is it possible to post photos? 

She is cow bred, so like a previous post said, she will more than likely stay that way. It wouldn't be as big as a deal if it's not 'severe'.