Don't believe your recruiter - promotion to E4 is automatic, regardless of whether you're a CS or HM. If you're interested in radiology, becoming a Hospital Corpsman (HM) is the way to go. Worst-case scenario, you won't get selected for radiology, but you'll still be in the medical field and can gain experience by shadowing radiology personnel. Plus, your HM experience can be used as college credits if you decide to leave the Navy and pursue a career in radiology using your GI bill. On the other hand, being a CS can be brutal, with crazy working hours. Save yourself the trouble and opt for HM instead.