I might not be the best to judge but here you go! I agree with the fact it depends on what you want to do with him (or her?), but I like him! Was he 4 yrs old in 2011 or is he 4 yrs old now? If 4 yrs now, he looks great for his age. You guys look like a good pair too!
If he's been 4 since 2012, then jumping isn't a problem. A horse can start jumping at four because that's the average age most horses' knees lock into place. My vet, who is also my former trainer, liked to keep horses jumping from 18"-2'3" their first year, depending on the horse. She didn't jump them more than twice a week and only upped the workload during their five year old year. If this video is of this horse late into its fourth year, I don't see a problem. However, if it's been jumping since it was a three year old, then there could be some serious issues with arthitis later on.
Regardless, a vet check is ALWAYS a must if its an unfamiliar horse and barn.
Well you can't really judge conformation from this video, but she's a cute little jumper. Average movement, I'd say, and I know she's still young but I would like to see her a little more balanced coming into some of the jumps. That being said, I think she'd make a nice lower-level eventer. She looks like she's jumping a bit flat, so if I were to go look at her I'd put her through some gymnastics to see just how much scope she has so you can get a bit of an idea of where she'll max out. I could see her easily going training level (3'3"), but from this video I can't see her being a great option for too much higher than that.
All in all, I think she's a cute horse. I wouldn't personally be interested in her unless she proved to be spectacular in person and it was just the video that didn't do her justice. So...yeah. That's my opinion, haha.