I have 4 houses on 1 sewer line, 4" pipe, that gets clogged frequently. All of the plumbers agree that the pipe is large enough and there should me no problems. I have to clean it out almost every other month with a sewer snake. Is there a chemical I can use for preventative maintenance?

There is no chemical better then a snake. Where do you snake from and how much snake do you put out? If it's root then I can help you there with chemicals.
If you wish to control roots RootX or Robics Foaming Root Killer that contain Dichlobrnic are two products you could try.
A less expensive way would be to call around to garden supply stores and ask for fine grain Copper Sulphate. Put a 1/2 pound in your toilet and flush it down. Repeat in 6 months.
Hope this helps and thank you for rating my answer, Tom