If you want to be a dentist, you need to apply to dental school first. I think you would be seriously delaying your career and handicapping yourself to do another enlistment if you are truly motivated to be a dentist. If you spent those years in dental school with a Navy scholarship, I believe those years would also be credited toward a military retirement, just as they would if you were serving an enlistment. And you would be getting a stipend and having your education paid for by the military at the same time. A mid-career dentist makes $200K+ in the civilian world and senior dental officers in the military make about $180K+, with all the bonuses, housing allowance, etc. You do the math. You would be receiving a free dental education, potentially worth around $160K, and getting a 4 or 5 year acceleration to earning over $100K a year for those years. Compare those numbers with what you would earn during those same 5 years as an E5 or E6 enlisted.
You already have 4 enlisted years. You would be receiving 4 more years credit for dental school if you get a scholarship. Then you would put in a 4 year payback on active duty as a dental officer. So you would have accumulated 12 years credit toward a 20 year retirement before you know it. Seems like a better way to go to me, but only you know your abilities and motivation.
If you become a dental officer you are crossing over from the enlisted side to the commissioned officer corps and it's a totally different book of rules. An enlisted person takes an oath to defend the Constitution and also obey the President and the chain of command; an officer takes an oath to support and defend the Constitution - it's a subtle but significant difference. I don't know if the Navy would release you from your enlisted commitment to go to dental school and become an officer, but you should talk to a Navy health corps officer recruiter to get that information. I would not ask an enlisted recruiter; he will probably tell you anything to get you to enlist.
Enlisted deployments? It all depends on your enlisted specialty. You are former enlisted and know your rating, so you should be able to answer that. Don't forget that dental officers deploy too. In general, the officer community is competitive, significantly more so than the enlisted community. If you have the qualifications to compete you will do fine. But you must apply, and get yourself accepted, to a dental school first. No one else is going to do that for you.
If you do not yet have the qualifications to apply to a dental school, dental school prerequisite courses can be difficult (impossible?) to acquire while you are on active duty. Unless things have changed since I went to college, you will probably not be able to get all of them while attending night school in an enlisted status.
