The thing is this.
You are new to horses, which means many of the problems you may run into with a 4 year old you will not have the experience to either recognize before they happen, be able to fix once they start happening, and can end up not just getting yourself hurt, but the horse too.
A young horse is much more likely to have issues of trying things, than an older and well trained horse, due simply to that horse's years under saddle.
That said, I've known young horses that never offered a moment's trouble BUT I grew up around horses, and have worked with them most of my life, and that makes the difference.
Much depends on the horse, but much depends on how you handle horses. I probably would advise against it, unless you had the money to hire trainer to work with you.
Horses make me a better person.