AP chem would be harder then normal chem; but since you already have covered basic chemistry it should not be too hard. Probably just a little more algebra. As if it will count for the 4 "science units", that depends on your school, and local codes and such. Just ask your teachers. And depending on your school, instructors, and funding the labs can be way way more fun and exciting; as well as educational!
If you like chemistry, and think you want to study it in college, take the AP chem, work really hard, and do good on the placement test and you can skip the first year of freshman chemistry if you want to (though it would not hurt you to take the first year of chem courses to get easy A's while you take some of the other required classes). Your choice.