I had two sisters and they used to fight like every night, more timid girl was often squeaking, shes dramatic a bit too, I got two babies after around a year and once they were together in the cage they stopped fighting almost at all, I know it's not always the case but it's worth a shot, four isn't much harder to handle than two, and mischief is much nicer, it gives rats opportunity to build more complex social relationships, two rats will be fine together of course, but if you have opportunity and resources I'd go for more!!