With four of you there will be a substantial savings. Just be sure you make a reservation. Most of the locals fly on what is called an 'open ticket' so if you have a reservation you would have priority. Allow at least one hour from the time you land at the international until your scheduled flight out of muni.
I think you will actually even enjoy the taxi ride into town. Don't be surprised if after you are on board your plane it takes off from Muni and lands at the international where it will fill any empty seats. The looks on the faces of the people who were on the flight with you will be priceless.

Harriette
Take only pictures leave only bubbles