Is there a difference between flying in a small 4-seater non-pressurized plane vs a commercial pressurized airplane for a pregnant woman of 20 weeks?

No: My understanding is that a pressurized commercial airliner is not like being at ground level, but more like being at 6000-7000 feet above sea level. Depending on how high you are flying in a non-pressurized plane, it might be around the same pressure. Usually non pressurized planes rarely go above 9000 feet.

Not Generally: Commercial Airplanes are pressurized to 10,000 ft altitude so they can go higher usually around 35,000 ft. A small plane will not go to those altitudes, thus they stay where pressurization and/or Supplemental Oxygen is not needed.