My own 4 year old hits sometimes when she is angry. She ONLY hits her sister and only when completely cornered (trust me her sister deserves it
). We discipline her out the wazoo but it still happens sometimes.
She has never even come close to doing this with any of the daycare kids or any of the kids in her preschool. It might be possibke this is purely situational (a learned behavior in a specific environment) so I would ask more questions and maybe do a trial run with her.
I also have two kids who were regular hitters (3.5 yr old) and pinchers (19 months old) and I nipped both in the bud with punishments for the behavior and getting parents on board with more consistant punishment.