When it comes the time, ask a recruiter. Policies change fairly frequently but as far as I know ROTC of any sort will only get you E3 because to be an E4 you have to be rated. Regarding being an officer, you have to go through the Coast Guard Academy or OCS to be an officer in the Coast Guard, not sure about other services. The Coast Guard doesn't even accept graduates from other maritime academies for direct commission, they still have to go through mandated training like everyone else.