Is it wise to be stern with children when they misbehave from ages 4-9? The children might perceive it to be an unwanted intrusion and may hurt them psychologically. Should one yell at children if they are disobedient with a parent and insist under penalty of severe punishment and in no uncertain terms demand obedience?

Is it Catholic to be harsh with the children in discipline?
  Please explain to me.

It is important to be disciplinary with children so they learn from their mistakes and don't repeat them as they grow older. That being said, spanking isn't the only way to use discipline, nor is it always the most affective. It depends on the child and the circuмstance. Yelling accomplishes practically nothing. It only either causes the child to keep doing whatever they're doing or upset/scare them.

I have a problem with yelling, or at least using a very, very harsh voice, with my daughter in certain situations. She's almost 3, but as young as 2 when I'd spoken to her very harshly. I've felt very bad afterwards, and occasionally I find myself speaking with a harsher tone than I'd like. I know my father would used to yell very very harshly growing up, and (not using it as an excuse) I think I've picked up that trait. I'm trying to drop it, as it takes much effort since I seem to do it automatically without thinking.

I do give her a hug after any type of punishment, and make sure she understands why she went on time out (for example). Then I tell her I love her, and that I know she's a good girl, but if she wants to stay a good girl she needs to listen to mommy and daddy (for example).

They forgot to give me the parenting book when my children were born lol jk jk