billing
Are you doing strictly insurance follow up? That is what I am assuming by your question.
If you are just doing follow up, the way an office I have worked in would do this:
Give each one a major carrier (3 billers had commercial, 1 biller was Medicare, and 1 was Medicaid) for a total of 5. Medicare and Medicaid were very time consuming and had many "special" rules, so they really need a dedicated person. Since you are 4 you could do 2 on commercial. And if the Medicare/Medicaid had lag time they can help with commercial. Then the numerous smaller carriers (less denials) were put into an alphabetical file (organized by carrier) and worked daily along with the assigned carrier.
If you are asking for both billing and follow -up, Greg's suggestions are a great way to go, depending on volume of course. You want to avoid the same person who does your payment posting from having any other "billing" duties. They should not be posting charges and posting payments. That can get very sticky, fast!
Are the 6 doctors seeing around the same number of patients per day? That will make a difference for each one to have a specific doctor and or major carrier. Obviously someone is going to have more than one doctor if that is the route you go. So, the others with a single doctor could also have a major carrier.
I hope this helps. Good luck.