I have two male cats with two female cats. The problem is that the older male cat is hurting his 6 month old male kitten.
What should I do to stop the older one from doing this ?
I suggest that you spay/neuter all 4 of them.
This will not only likely solve the aggression problem but also prevent the problem of producing about 8 kittens twice a year.
That's 16 kittens per year on average in the first year. Then the next year each of the kittens, say half of them are female, will produce 16 kittens itself. That's 16 kittens from the first two mother cats plus 8*16=128 kittens from the second generation cats.
That's 144 cats in year two. And so on.
If you love cats do the world some good and pay for surgery in order to prevent future unnecessary suffering.
You should neuter the adult male cat. Then you should begin behavior modification as suggested by keshlam in the comment that he should be creating as an answer soon.