we are also working on this with our 4-yr-old daughter. she is kind to every other animal she meets (to the point that people comment on her kindness and animals gravitate towards her), but she has difficulty with our cat. i have consulted developmental psychologists about this, as it is the one difficult behavior that we haven't been able to curb completely. and this is what we have learned:
yes, 4-year-olds are teachable about kindness, but also impulsivity is out of their control. the cortex understands the rules but their limbic brains don't care yet. so it is a contest between restraint and yielding to impulses, and as of now, the cortex cannot overrule limbic brain to manage impulses. by age 8, the cortex should have more control. our 4-yr-old, who has difficulty being nice to the cat is not a psychopath in training.
so what we have been doing is having her rehearse being kind to the cat. we tell her: "one of our jobs as parents is to teach you. and we are teaching you a value that is important in our house. this value is being kind to every creature, including the cat. what does it mean to be kind to the cat? how can you be kind?" and then she lists ways to be kind to the cat. then we ask her what are mean things to do to the cat? and she lists those. and then we ask her: do we do those mean things? she says no. and we ask her EVERY DAY and have this conversation with her EVERY DAY now. if she is mean to the cat, then she gets a time out away from the cat.
and this is how we "reward" her for being kind-- both in physical practice and in her ability to TELL us how to be kind. we compliment her on her kindness and are very specific ("i see how you are scritching the cat's neck so softly. she really likes that"). also, our daughter loves trains and matchbox cars, but yesterday she was mean to the cat, so i took them all away. right now, if she she both practices kindness and explains to us the value of being kind for one day, she gets one train and one piece of train track. so this will be a long, daily practice (she's the youngest of her cousins, so she has a lot of hand-me-down trains/cars :). if she is mean to the cat or refuses to explain the value of kindness in our household, then she gets the time out (for physical harm), we reiterate the values, tell her that everyone makes mistakes every now and then (remember, it really isn't in their control at their age: would you have a 4-year-old babysit or house sit or dog sit on his/her own?), and that we will try again tomorrow. and she doesn't get the one train and piece of track for that day. she knows what she did is wrong.
she needs to learn the value of kindness; we need to teach her the value. and yes, as long as the cat has a safe place to be in the house and we are on top of the behavior, the cat can remain. i am an avid animal rights believer and would never keep our cat in a harmful situation. but this is something that can be worked on.
good luck, mama! keep us posted.