Aggression or playing?

Both are kittens, and kittens play rough. Right now, what is going on is which kitten will be the alpha kitten.

Seems like simply play to me, and don't be worried if they seem to scream out. Often that is bluff. Rarely is any blood drawn, and ears back before they do a boxing match is common. It is serious games they are playing and it is all part of life.

My youngest kitten seemed to be picked on by an older kitten. She would scream bloody murder. Then, within seconds, she was giving it to the older kitten as hard as she got it. Yes, play. And kittens play rough.

There can by only one alpha kitty in a home, and they are working out which one that will be. And your 9 week old kitten will be doing some pretty fast growth, which will be a great equalizer.

How about Either, nether or both??? If your cat were angry or trying to do damage you would know. The snarling and hissing would be a big tip off and the clumps of fur on the ground would be a dead give away. The bruises and cuts on the kitten would also announce the deed.

I think they are going through a shake down in which they decide who has what and who is top cat.

Make sure neither is being physically injured and let them work it out.

Be sure each has a food and water bowel that is just *His or hers* and a litter box that is personal to each specific cat. See if that helps them get along.

Normal kitty behavior. If you notice spraying or peeing out of the litter box, this is a problem or if your cats are not spayed or neutered then this can also cause problems, but your kitties are probably getting to know eachother in their own weird way.

Remember, cats are not pack animals like dogs and it takes time for kitties to warm up to eachother. They are both really young so it sounds like they are just playing and trying to figure out who will be the alpha cat (top cat) in your house.

Are the four-month old's ears back, is his tail wagging in a threatening manner, or is he making a growling/moaning sound when he is doing this? If two of the three are present, it is likely aggression.

If there are the normal pissed off kitty sound then they just might not like each other, or she is scared, if her ears are laid back, and her tail is thrashing around wildly, I would say it could be aggression