My kittens are seven months old now and are very close and loving to each other. Do they usually stay best friends their whole lives or do they grow apart when they get older? They are two girls and two boys.
Chuckles
My son in law took in two cats that were sisters from the same litter and they were extremely close until one of them passes last year. This is just an anecdote but what I have seen is indeed the case.
ScienceTime
As long as they are all fixed/neutered, the males don't usually fight. They usually get along as long as no one in the household is pitting them against one another such as with competition for food.
Giving them treats only when they are all together helps, giving them places to sleep where they can sleep in a group and places where they can sleep alone when they want to, that all helps.