Usually the breakdown for LoRs go like this:
2 university professors (science)
1 dentist
1 misc (family friend, research/internship mentor, other professor, etc.) - optional
Some schools don't even make it mandatory to get one from a dentist, so just make sure when you look at the criteria at each school you're applying to
Most schools only require 3 letters (preferably in the form of a committee letter if your undergrad offers one), but it won't hurt to get a 4th if you feel that your neighbor will write you a really strong recommendation