Aside from the MD, there are few differences between the two types of programs.
4-year residents typically spend 30-36 months on the OMFS service (the remaining 12 - 18 months are spent on Anesthesia, Medicine, ENT, Plastics, General Surgery, ER, etc.: it varies from program to program).
6-year residents typically spend 30-33 months on the OMFS service (sometimes the med school years include some OMFS rotations - be sure to include these when looking at programs). A friend of mine in a six-year program had the following schedule:
Intern Year: 12 Months OMFS
MS3: 12 Months Medical School
MS4: 8 Months Medical School, 4 Months of Anesthesia at Resident Level
General Surgery: 12 Months, split between Cardiac, Trauma and Plastics
Junior Resident Year (12 Months): 9 Months OMFS, 3 Months ENT
Senior Resident Year (12 Months): 12 Months OMFS
Total OMFS Time: 33 months OMFS
So, as you can see, there is little difference between the total time on OMFS between these two types of programs (granted that there are some 4-year programs that may spend much more time on OMFS than some 6-year programs, but the difference is only a few months for most programs).........