Is a young horse too risky for first purchase?

Honestly, I don't think it is a smart idea. If your looking to purchase a horse for the first time, get a "been there, done that" school master. I gave my friend her first horse, a 26 year old Appy, he taught her SO much that is essential for a first time horse owner.

If you decide to get a 4 year old horse, would you be riding and training with a trainer? Do you have much riding experience?

It really does depend a lot on the temperament of the horse and how well trained it already is and how much time its had under saddle. I would also say that having a good trainer on speed dial makes a huge difference, sometimes a placid young horse that's picked up no bad habits can work better if you're prepared to listen to good advice from someone 'hands on' as soon as you feel a problem coming along
I've had young horses that have never put a foot wrong and no more 'frisky' than anything older
I've also had older horses that have been complete idiots because they've belonged to complete idiots.

I am very sorry for your husband's suffering.... it sounds really painful. I hope that the OP goes with what is best for him/her. The best choice is probably a SANE, well broke older horse. I think that is true for a majority of situations, to be honest. Just because a person is "experienced" with horses does not mean they will be able to deal with one who is problematic. The same goes for any animal....the danger with horses is that they are SO MUCH BIGGER than us.

I bought my filly because I was drawn to her and impressed by her unflappable personality. Sure, she has her moments, but I will not rush her or myself...plus it really helps that she is a total and complete slug at heart. XD If I do not feel ready to ride her once she is broke to ride, I would be willing to buy another horse, so as to not risk harm to either of us. Not everyone is prepared to make that choice...