4 is young. I agree with those who say she has a lot of growing and changing to do still. I bought my draft cross when she had just turned 5. She seemed fine when I bought her, and well balanced. She needed some muscle tone but she wasn't running down hill and her shoulder was relatively petite. A few months after I bought her she became super butt-high, and lop-sided. I had to have corrective hoof-trimming done because one hind leg was suddenly longer than the other. All I could think was "Oh no! What did I just buy?!" Thankfully, she only took another 8 months or so to level out again. She didn't reach her current height until she was almost 7 years old. She was 16.2 hands when I bought her as a 5 year old and now she measures between 17 and 17.1. I have found that the big guys can be late bloomers and a lot can change between 4 and 8 years of age.
Her general structure should remain the same, but I think her shoulder will catch up and she should level out. For being green I don't think she's tracking up too badly. There's no telling what some proper riding and conditioning, and some time to mature will do for this mare. She's already not a bad looking horse and time and conditioning will only improve things.
Though you do need to consider your goals. I don't think this mare is super high caliber. Watching the video, she's fairly average in the ways he moves. She certainly won't be taking you to grand prix, and if you have high aspirations then a well-bred warmblood with a family history of successful dressage mounts may be a better option, but if you are content to have fun at the lower levels and see how far this horse takes you, I think you should go for it!