Dental Hygiene?

If you are going to get a 4 year degree I would tell you to try to go to dental school and not settle for dental hygiene. My suggestion is that you shoot for the top of the heap in whatever endeavor you are undertaking.

If you state only has an AS then you could do that then finish a BS online DH completion degree. It depends on what you want to do with your degree thereafter. If you want to just practice as a clinician private practice, it doesn't matter if you have an AS or BS; they salary is no different. If you are going to maybe do sales or teach in a DH program in the future, then it is best to further your education.
The other option is what you suggested: Do a 4 year degree then go into the DH program to get your AS in DH. The programs are highly competitive in getting in and each program has different requirements to get in, so make sure you have all your bases covered for your pre-requisites. I have seen some students go and start a BS degree with dental hygiene in mind with pre-requisites and if they do not get into the program, they continue their under grade BS until they get into the program. If they get in while they are in under grade, they go to the DH program, complete that, then go back and finish their BS.
I personally went and started an under grade in teaching, got into DH and didn't go back till almost 18 years later and finished my BS in dental hygiene. So my encouragement is, don't worry too much about it because there are a lot if completion programs online whenever you choose to go back! School will always be there when you want to go back. My desire to go back was just for myself accomplishment. Also, I always wanted to go back, and give back to academia to teach DH.
Best to you! Hope I helped some!