Hi,
Can you get letters from your science professors at your university or college? If you can, i strongly suggest you to do so. A letter from a teacher that you had in 6th grade will not help you with the admission committe. 4 Letters from 4 different dentists do not help either. Most schools require only 1 letter from a dentist, and EMPHASIZE significantly on letters from science professors from university or college. If you do not have a pre-dental committe, I suggest you should ask for 3 letters from 3 science professors to substitute that (if you read the requirement from each school that you are going to apply, this would be case: 3 LOR from science professors = 1 LOR from pre-dental committee).
In my opinion, your choices of LOR are weak and will not meet the LOR requirement for most dental schools. The dental schools do not mind if you send more than what they require, but they specifically check the letters and make sure their requirement is fulfilled. For an instance, lacking in 1 letter from a biology professor will make your profile incomplete (this happens to my friend at a certain school).
Good luck!