Your opinions about this?

Well, if you feel like he's the one, than go for it!! Just pay attention to his age. Are you more of a beginner? Or more advanced? If you're more on the beginner side, than I'd recommend an older horse that's been there, done that. As far as him being too young to start dressage, I don't think he'd be too young. Actually, I think the earlier, the better. If he knows the basics already and is comfortable with what he's doing, than I'd say go for it and start training! But if he's not as solid with the basics yet, than teaching him dressage will just confuse him and could make him lose the basic stuff. I'm more of a western rider, so I'm just basing it on how old we start are horses with training for whatever type. And we start rather early. 4 years should be old enough though. Again, in my opinion it's more about how experienced the horse is or not. It helps that you have a trainer, too! Hope this helps!